Math into LATEX: An Introduction to LATEX and AMS-LATEX

Progress in Mathematics Volume 241

Series Editors H. Bass J. Oesterlé A. Weinstein

José Seade Andrea Malchiodi

On the Topology of Isolated Singularities in Analytic Spaces

Birkhäuser Verlag Basel
Boston
Berlin

Author: José Seade Instituto de Matemáticas Unidad Cuernavaca Universidad Nacional Autónoma de México (UNAM) Av. Universidad s/n, col. Lomas de Chamilpa, Ciudad Universitaria C.P. 62210 Cuernavaca, Morelos México e-mail: [email protected]

2000 Mathematics Subject Classification Primary 14Bxx, 32Sxx, 53Cxx, 55A25, 57C45; Secondary 11Fxx, 32L30, 34Mxx, 37F75

A CIP catalogue record for this book is available from the Library of Congress, Washington D.C., USA

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ISBN 3-7643-7322-9 Birkhäuser Verlag, Basel – Boston – Berlin This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. For any kind of use whatsoever, permission from the copyright owner must be obtained. © 2006 Birkhäuser Verlag, P.O. Box 133, CH-4010 Basel, Switzerland Part of Springer Science+Business Media Printed on acid-free paper produced of chlorine-free pulp. TCF ∞ Printed in Germany ISBN-10: 3-7643-7322-9 e-ISBN: 3-7643-7395-4 ISBN-13: 978-3-7643-7322-1 987654321

www.birkhauser.ch

To Swami Chidvilasananda and the Siddha lineage, whose light turned my life into a joyful path; and to Teresa, whose love is my everything.

Preface Singularity theory stands at a cross-road of mathematics, a meeting point where many areas of mathematics come together, such as geometry, topology and algebra, analysis, differential equations and dynamical systems, combinatorics and number theory, to mention some of them. Thus, one who would write a book about this fascinating topic necessarily faces the challenge of having to choose what to include and, most difficult, what not to include. A comprehensive treatment of singularities would have to consist of a collection of books, which would be beyond our present scope. Hence this work does not pretend to be comprehensive of the subject, neither is it a text book with a systematic approach to singularity theory as a core idea. This is rather a collection of essays on selected topics about the topology and geometry of real and complex analytic spaces around their isolated singularities. I have worked in the area of singularities since the late 1970s, and during this time have had the good fortune of encountering many gems of mathematics concerning the topology of singularities and related topics, masterpieces created by great mathematicians like Riemann, Klein and Poincar´e, then Milnor, Hirzebruch, Thom, Mumford, Brieskorn, Atiyah, Arnold, Wall, Lˆe D˜ ung Tr´ ang, Neumann, Looijenga, Teissier, and many more whose names I cannot include since the list would be too long and, even that, I would leave aside important names. My own research has always stood on the shoulders of all of them. In taking this broad approach I realize how difficult it is to present an overall picture of the myriad of outstanding contributions in this area of mathematics during the last century, since they are scattered in very many books and research articles. This work is a step in that direction and our first main purpose is to put together some of these gems of current mathematics in an accessible way, indicating in all cases present lines of research and appropriate references to the literature. This step occupies Chapters I to V of this monograph. I have made some contributions to the subject, either by myself or in collaboration with other colleagues, and I think they are interesting enough to be included here. On the other hand, the theory of real singularities is much less developed than that of complex singularities, for various reasons. There are however interesting families of real analytic singularities having remarkable geometric and topological properties similar to those of complex singularities. These are related to complex differential equations and defined via complex geometry, and the second main

xiv

Preface

purpose of this monograph is to explain some aspects of their geometry and similarities with complex singularities. I believe the study of these singularities in Chapters VI–VIII and the picture of complex singularities presented in Chapters I–V, together provide interesting new insights into the geometry and topology of singularities. This study also gives new methods for constructing smooth differentiable manifolds equipped with a rich geometry and defined by algebraic equations, which can be of interest on its own. For instance, at the end of Chapter VI we obtain explicit algebraic equations that provide analytic embeddings of oriented surfaces of all genera in the sphere S3 ; this can be useful for problems in differential geometry and physics. The material that I present here is mostly contained in the literature but, as said before, it is spread over many different research papers and books, making it difficult to put together a complete picture. In some cases I have given simple proofs of well-known theorems; in other cases I have included the complete proofs in detail; yet in others I have preferred to content myself by giving only some of the key ideas to understand some specific theory. In all cases I have included appropriate references and indications for further reading. I have also added a number of open problems as extension of the text. This monograph is constructed like a fan, where each individual chapter is to some degree independent of the others and can be read on its own, and yet as a unity, spreads out to give the reader a taste of the richness of this fascinating topic. I am sure that everyone who reads it, regardless of whether he or she is a graduate student, an expert in singularity theory, or a researcher in any other field of geometry and topology, will find interesting things in it.

Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

I

A Fast I.1 I.2 I.3 I.4 I.5 I.6 I.7 I.8

II

Trip Through the Classical Theory An example: the Pham-Brieskorn polynomials . . . . . The local conical structure . . . . . . . . . . . . . . . . Ehresmann’s fibration lemma . . . . . . . . . . . . . . . Milnor’s fibration theorem for real singularities . . . . . Open book decompositions and fibred knots . . . . . . On Milnor’s fibration theorem for complex singularities The join of Pham and the topology of the Milnor fibre. The Milnor number . . . . . . . . . . . . . . . . . . . . Exotic spheres and the topology of the link . . . . . . .

Motions in Plane Geometry and the 3-dimensional Brieskorn Manifolds II.1 Groups of motions in the 2-sphere. The polyhedral groups . . . . . . . . . . . . . . . . . II.2 Triangle groups and the classical plane geometries . . II.3 The 3-sphere as a Lie group and its finite subgroups . II.4 Brieskorn manifolds and Klein’s theorem . . . . . . .
II.5 The group P SL(2, R) and its universal cover SL(2, R) II.6 Milnor’s theorem for the 3-dimensional Brieskorn manifolds. The hyperbolic case . . . . . . . . . . . . . II.7 Brieskorn-Hamm complete intersections. The theorem of Neumann . . . . . . . . . . . . . . . . II.8 Remarks . . . . . . . . . . . . . . . . . . . . . . . . .

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11 14 17 18 21 22

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25 30

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36 42 47 50 55

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57

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60 62

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x

III

IV

V

Contents

3-dimensional Lie Groups and Surface Singularities III.1 Quasi-Homogeneous surface singularities . . . . . . III.2 3-manifolds whose universal covering is a Lie group III.3 Lie groups and singularities I: quasi-homogeneous singularities . . . . . . . . . . . III.4 Lie groups and singularities II: the cusps . . . . . . III.5 Lie groups and singularities III: the Abelian and E + (2)-cases . . . . . . . . . . . . . III.6 A uniform picture of 3-dimensional Lie groups . . . III.7 Lie algebras and the Gorenstein property . . . . . . III.8 Remarks . . . . . . . . . . . . . . . . . . . . . . . .

65 71

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75 80

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82 85 87 88

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92

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97

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Within the Realm of the General Index Theorem IV.1 A review of characteristic classes . . . . . . . . . . . . . IV.2 On Hirzebruch’s theorems about the signature and Riemann-Roch . . . . . . . . . . . . . . . . . . . . IV.3 Spin and Spinc structures on 4-manifolds. Rochlin’s theorem . . . . . . . . . . . . . . . . . . . . . IV.4 Spin and Spinc structures on complex surfaces. Rochlin’s theorem . . . . . . . . . . . . . . . . . . . . . IV.5 A review of surface singularities . . . . . . . . . . . . . IV.6 Gorenstein and numerically Gorenstein singularities . . IV.7 An application of Riemann-Roch: Laufer’s formula . . . IV.8 Geometric genus, spinc structures and characteristic divisors . . . . . . . . . . . . . . . . . . . IV.9 On the signature of smoothings of surface singularities IV.10 On the Rochlin µ invariant for links of surface singularities . . . . . . . . . . . . . . . . . . . IV.11 Comments on new 3-manifolds invariants and surface singularities . . . . . . . . . . . . . . . . . . On the V.1 V.2 V.3 V.4 V.5

VI

. . . . . . . . . . . .

Geometry and Topology of Quadrics in CPn The topology of a quadric in CPn . . . . . . . The space CPn as a double mapping cylinder . The orthogonal group SO(n + 1, R) and the geometry of CPn . . . . . . . . . . . . . . Cohomogeneity 1-actions of SO(3) on CP2 and The Arnold-Kuiper-Massey theorem. . . . . .

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105 110 116 121

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. . . . . . . . . 138 . . . . . . . . . 141 . . . . . . . . . 143 S4 . . . . . . . 148 . . . . . . . . . 150

Real Singularities and Complex Geometry VI.1 The space of Siegel leaves of a linear flow . . . . . . . . . VI.2 Real singularities and the L´ opez de Medrano-Verjovsky-Meersseman manifolds . . . . . . . . VI.3 Real singularities and holomorphic vector fields . . . . . VI.4 On the topology of certain real hypersurface singularities

. . . 157 . . . 163 . . . 167 . . . 170

Contents

VII Real Singularities with a Milnor Fibration VII.1 Milnor’s fibration theorem revisited . . . . . . . . . . VII.2 The strong Milnor condition . . . . . . . . . . . . . . VII.3 Real singularities of the Pham-Brieskorn type . . . . VII.4 Twisted Pham-Brieskorn singularities and the strong Milnor condition . . . . . . . . . . . . . . . VII.5 On the topology of the twisted Pham-Brieskorn singularities . . . . . . . . . . . . . . VII.6 Stability of the Milnor conditions under perturbations VII.7 Remarks and open problems . . . . . . . . . . . . . .

xi

. . . . . 175 . . . . . 177 . . . . . 181 . . . . . 187 . . . . . 191 . . . . . 194 . . . . . 196

VIII Real Singularities and Open Book Decompositions of the 3-sphere VIII.1 On the resolution of embedded complex plane curves . . . VIII.2 The resolution and Seifert graphs . . . . . . . . . . . . . . VIII.3 Seifert links and horizontal fibrations . . . . . . . . . . . . VIII.4 An example . . . . . . . . . . . . . . . . . . . . . . . . . . VIII.5 Resolution and topology of the singularities z1p z¯2 + z2q z¯1 = 0 . . . . . . . . . . . . . . . . . . . . . . . . VIII.6 On singularities of the form f g¯ . . . . . . . . . . . . . . . .

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200 204 206 209

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Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

Introduction Chapter I sets up the foundations for the rest of the book. Here we review some of the classical results of Milnor and others about the topology of real and complex analytic spaces around an isolated singular point. Two of these results are key for us. One is the theorem about the local conical structure of analytic sets. This has a long history, starting with Brauner (1928) and others in their local study of complex plane curves, noticing that every complex curve in C2 with an isolated singularity at the origin meets transversally every sufficiently small sphere around 0; the intersection is a classical link in the sphere (i.e., a collection of pairwise disjoint embedded circles), and therefore knot theory can be used to study complex plane curves. In 1961 Mumford proved a very surprising result in this direction: he showed that one cannot find a counter-example to the classical Poincar´e conjecture using complex singularities; more precisely he proved that if the link (i.e., the boundary of a small neighborhood) of a point in a complex surface is simply connected, then the point is actually smooth and the link is the usual 3-sphere. May I recall at this point that a few years earlier (1956), Milnor had found the first examples of exotic differentiable structures on spheres. Hence these two theorems (of Milnor and Mumford) drove the attention of several great mathematicians towards studying the topology of links of isolated singularities in analytic spaces. In his classical book [168] Milnor proved that if V is a (real or complex) analytic space in An , where A is either RN or CN , and if V has an isolated singular point at the origin, then V intersects transversally every sufficiently small sphere Sε (0) around 0; the intersection M = V ∩ Sε (0), the link, is a smooth manifold of real dimension m − 1, where m is the real dimension of V , and the topology of V and of its embedding in An are completely determined by the embedding M
→ Sε (0); more precisely, the pair (Bε (0), V ∩ Bε (0)) is homeomorphic to the cone over the pair (Sε (0), V ∩ Sε (0)), where Bε (0) is the closed ball bounded by Sε (0). The manifold M is the link of 0 in V . The second result in Chapter I which is key for this work is the fibration theorem of Milnor. The first part of it holds for real singularities in general and is just an easy extension of Ehresmann’s fibration theorem. This says that if a real analytic map f : (Rn+k , 0) → (Rk , 0) is a submersion at every point in a punctured neighborhood of the origin, then for every sufficiently small sphere Sε (0) around 0,

2

Introduction

and for every δ > 0 sufficiently small with respect to ε, the restriction of f to the )∩Bε (0), where Sk−1 is a sphere in Rk around 0, is the projection of tube f −1 (Sk−1 δ δ a (locally trivial) smooth fibre bundle; and moreover, the tube f −1 (Sk−1 ) ∩ Bε (0) δ can be “inflated” to become (diffeomorphically) the complement of the link M in Sε (0), and therefore one has a fibre bundle projection Sε (0)− M −→ Sk−1 . This is, δ so to say, the “weak” form of Milnor’s fibration theorem (see [167]). The strongest form of this theorem is for complex singularities: if f is now a complex analytic map (Cn+1 , 0) → (C, 0) with a (possibly non-isolated) criticalpoint at 0, then the above projection map Sε (0) − M −→ S1δ is given by φ = f |f |. If 0 ∈ Cn+1 is an isolated critical point of f then one gets an open book decomposition of the sphere (see the text for the definition), where the binding is the link M and the pages are the fibres of φ; moreover, each fibre Fθ = φ−1 (eiθ ) has the homotopy type of a bouquet of spheres of middle dimension. A corollary of Milnor’s work is that for these singularities the link is a highly connected manifold which bounds a parallelizable manifold (the Milnor fibre). We include here a result of [201, 269] extending Milnor’s fibration theorem to the case of meromorphic functions f /g. This is related to the results in Chapter VIII. The prototype of the singularities to which Milnor’s theorem applies are the famous singularities: f (z0 , . . . , zn ) = z0a0 + · · · + znan ; ai ≥ 2 , i = 0, . . . , n , studied by Pham and Brieskorn. Inspired by his knowledge of physics, where it was customary to think of the tangent bundle of the n-sphere as given by the equation z02 + · · · + zn2 = 1, Pham proved in 1965 that given any polynomial f as above, its non-singular level z0a0 + · · · + znan = 1 contains a canonical polyhedron of real dimension n, now called the join of Pham, which is a deformation retract of it. This implies, in particular, that this hypersurface has the homotopy type of a bouquet of spheres of middle dimension: a strong indication pointing towards Milnor’s theorem about the topology of the fibres, proved a couple of years later. On the other hand Brieskorn proved in 1966 that in many cases the links of these singularities were exotic spheres, i.e., manifolds homeomorphic but not diffeomorphic to the usual 2n − 1-sphere, n > 3. There were remarkable generalizations of this result by Brieskorn himself and by Hirzebruch, proving (among other results) that every homotopy sphere that bounds a parallelizable manifold is the link of an isolated complex hypersurface singularity. In particular all the 28 different differentiable structures on S7 can be obtained in this way. Chapters II and III are a joint piece. These are about the beautiful relation between 3-dimensional Lie groups and 2-dimensional complex singularities. This began with Klein’s theorem in 1884 giving a relation between the finite subgroups of SU (2) and certain surface singularities in C3 . In the simplest case, Klein’s theorem establishes an isomorphism between Zp \C2 and the singularity z12 + z22 + z3p = 0, where Zp denotes the cyclic group of order p; hence the link of the latter is Zp \S3 ; in particular this proved that the group SO(3) ∼ = (±1\S3 ) of motions of 2 2 the Euclidean plane is the link of the quadric z1 + z2 + z32 = 0, so it is the unit

Introduction

3

tangent bundle of the 2-sphere; for the binary icosahedral group Γ = 2, 3, 5 one gets that Poincar´e’s homology sphere Γ\S3 is the link of z12 + z23 + z35 = 0, and so on. We recall that Klein’s program was to study geometry by looking at the corresponding groups of isometries of spaces. This theorem of Klein was later completed by Milnor, Hirzebruch, Neumann and Dolgachev, giving the complete classification of the 2-dimensional, isolated complex surface singularities whose link (i.e., the smooth boundary of a small neighborhood of the singular point) is of the form Γ\G, where G is a 3-dimensional Lie group and Γ is a discrete subgroup of G with compact quotient (these are called uniform subgroups). Actually their results, together with the classification of 3-manifolds of the form MΓ = Γ\G, where G and Γ are as above, i.e., a 3dimensional Lie group G with a uniform subgroup Γ, give a very nice, unified view of these manifolds. There are, up to isomorphism, six 3-dimensional, simply connected Lie groups with uniform subgroups Γ ⊂ G. Given any such group G and a uniform subgroup Γ, there is associated to G a canonical complex 2-dimensional manifold X, equipped with a canonical holomorphic 2-form and a foliation F of X defined, in all cases but one, by an action of G. The manifold X is in all cases given by an automorphy factor of some line bundle. The quotient VΓ∗ = Γ\X is a complex manifold, foliated by copies of Γ\G, with a canonical never-vanishing holomorphic 2-form. This manifold VΓ∗ is actually an open cylinder Γ\G × (0, 1), and one of its ends can be compactified by attaching to it a smooth divisor SΓ , so that we get a complex analytic surface VΓ = VΓ∗ ∪ SΓ which may have isolated, normal singularities at SΓ . In four of the six cases in question, this divisor can be blown down complex analytically and the result is a complex analytic surface VΓ with a normal singularity P , whose link is the 3-manifold MΓ ; these are the cases envisaged by Klein, Milnor, Hirzebruch, Dolgachev and Neumann. In the remaining two cases the divisor SΓ can only be blown down real analytically, so the quotient VΓ is (homeomorphic to) a 4-dimensional real analytic space with an isolated singularity P , whose link is MΓ and which has a complex structure away from P . In all cases one has on VΓ∗ = VΓ − P a canonical never-vanishing holomorphic 2-form that defines a trivialization of the tangent bundle of the link, and in all cases but one, we know that this trivialization lifts to a basis of the Lie algebra of the corresponding group G. I think this must also hold in the remaining case, but I do not know how to prove it. The goal of Chapter IV is to show some of the ways that the general index theorem of Atiyah-Singer has had impact in singularities theory. Mostly I restrict the discussion to two “particular” cases of the index theorem: the Riemann-Roch formula and the Hirzebruch signature theorem. We consider also Rochlin’s signature theorem. The general philosophy is the following. The index theorem of Atiyah-Singer may be thought of as a beautiful and far-reaching generalization of the HirzebruchRiemann-Roch theorem, both in statement and in the spirit of the original proof. Given a closed, oriented manifold M , vector bundles E and F over M and an elliptic operator D from the sections of E to those of F , one has that both the

4

Introduction

kernel and the cokernel of D are finite-dimensional, and the difference of these dimensions is by definition the analytic index of D. The index theorem gives a description of this integer in terms of topological data implicit in the elliptic operator, the so-called topological index. This establishes a very deep connection between analysis/geometry and topology. Special cases are the signature theorem of Hirzebruch, the Hirzebruch-Riemann-Roch theorem, the Lefschetz fixed point  formula, the relation of the Dirac operator with the A-genus for spin manifolds and several other fundamental theorems in mathematics. Rochlin’s signature theorem can also be seen through index theory (see Sections 3 and 4 in Chapter IV). If (V, 0) is an isolated (real or complex) singularity germ in some affine space, then the diffeomorphism type of its link M = V ∩ Sε depends only on the analytic structure of V , and not on the choices of defining equations for V or the radius of the sphere. Thus any invariant of closed manifolds gives automatically an invariant of singularities. This has led to a myriad of interesting results for complex singularities, using invariants coming from the Hirzebruch-Riemann-Roch theorem, or one of the signature theorems mentioned above (Hirzebruch or Rochlin). For instance the formula of Laufer (and its generalization to higher dimensions by Looijenga) for the Milnor number, and that of Durfee for the signature, are both obtained in this way, and we review these in the text. We also give in this chapter a result of [76], which is a variant of Rochlin’s signature theorem for the case when the spinc manifold is actually a complex manifold. This theorem has the advantage of fitting naturally in the setting of algebraic geometry. Then, following [76], we give an interpretation of the geometric genus of normal, Gorenstein surface singularities in terms of the dimension of the space H 0 (−K, O), where K is the canonical divisor of the minimal resolution. This is interesting since the geometric genus is one of the key invariants for complex singularities, and it is related with the Seiberg-Witten invariants of the link (by [181, 182]). Chapter V somehow provides a higher-dimensional analogue of Klein’s theorem (in Chapter II) for the particular case of the quadric; this is based on [135]. We also prove an equivariant version of the Arnold-Kuiper-Massey theorem, saying that CP2 modulo conjugation is the 4-sphere; our proof is entirely analogous to that of Atiyah-Berndt in [17], though we only envisage the case of the complex projective plane and not the more general one that they consider. This is actually a byproduct for us, since our goal is to understand the geometry and topology of  where Q  is the quadric z 2 + z 2 + · · · + z 2 = 0. In fact Klein’s the pair (Cn+1 , Q), 0 1 n theorem gives very precise information about the quadric z12 +z22 +z32 = 0 using the group SO(3). Here we look at the canonical action of the special orthogonal group SO(n+ 1, R) on Cn+1 and on CPn , the complex projective space, in order to study  This is of course related to the classical problem studied by the pair (Cn+1 , Q). Zariski [267] and others, of describing the topology of the complement of an affine algebraic hypersurface V ⊂ Cn+1 . We actually look at the projectivized situation. We begin by showing that the complement of a non-singular hyperquadric Q in CPn is diffeomorphic to the total space of the tangent bundle of the real projective

Introduction

5

n-space RP n ,

CPn − Q ∼ = T (RP n ) .

Then we use the above observation on the topology of CPn − Q to describe CPn as the double mapping cylinder of the double fibration:

Q

F+n+1 (2, 1) II v IIπ2 π1 vvv II v v II vv I$ v { v

PRn

where F+n+1 (2, 1) ∼ = SO(n + 1, R)/(SO(n − 1, R) × (Z/2Z)), is the partial flag manifold of oriented 2-planes in Rn+1 and non-oriented lines in these planes. The manifold F+n+1 (2, 1) is diffeomorphic to the unit sphere normal bundle of Q in CPn , and it is also diffeomorphic to the unit sphere tangent bundle of PRn . This decomposition is related to previous work by V. Vassiliev [254], J. Tits, C.T.C. Wall [262] and others, and we refer to [135] for details. Then we look more carefully at the decomposition of CPn arising from the above double fibration. This describes CPn as a 1-parameter family of codimension 1 submanifolds F+n+1 (2, 1) × {t}, for t ∈ (0, 1), together with two “special” fibres: Q and a copy of the real projective space. We prove that these are the orbits of the natural action of SO(n + 1, R) on CPn , regarded as a subgroup of the complex orthogonal group SO(n + 1, C). This is an isometric action with respect to the Fubini-Study metric on CPn , and the principal orbits are the flag manifolds F+n+1 (2, 1), which have codimension 1. The space of orbits is the interval [0, π2 ]. The endpoints of this interval correspond to the two exceptional orbits, which are the quadric Q and the real projective space Π which is the fixed point set of the complex conjugation in CPn . Finally, in the last section of Chapter V we restrict the discussion to the case n = 2 and prove the equivariant Arnold-Kuiper-Massey theorem by constructing, via linear algebra, an explicit algebraic map CP2 → S4 which is SO(3)-equivariant and commutes with complex conjugation in CP2 . The topology of isolated complex singularities has been long studied by many authors, as we know already, and it has a beautiful and well-developed theory. Chapters I–IV above are a sample of this, while in Chapter V we use real analytic group actions and foliations in order to study complex geometry. Chapter VI is the turning point, as thereafter we are concerned with the real counterpart of this theory, largely inspired by [168, 251]. The theme in Chapters VI–VIII is the interplay between complex geometry and real analytic singularities. Let us explain this with more detail. In the last chapter of his book on singularities Milnor studied (germs of) real f

analytic mappings (Rn+k , 0) −→ (Rk , 0) which are submersions on a punctured neighborhood of the origin. For short we say that a map like this satisfies the Milnor condition at 0. He showed that such maps also define fibre bundle projections φ : (Sn+k−1 − M ) −→ Sk−1 ,

6

Introduction

where Sn+k−1 is a small sphere around 0 ∈ Rn+k , M = f −1 (0) ∩ Sn+k−1 is the link and Sk−1 is a small sphere around 0 ∈ Rk . The “weakness” of this theorem, as Milnor points out himself in his book, is that it is very hard to find examples of singularities satisfying these conditions, since the generic situation for k > 1 is to have a discriminant locus of dimension > 0 of points in the target over which the rank of the derivative drops. In fact Milnor asked whether there exist non-trivial examples of such maps (even for k = 2), other than the examples he gave in his book. This question was partially answered by Looijenga in 1971, and then by Church and Lamotke in 1975 used Looijenga’s technique, to give the complete classification of the pairs (n, k) for which there exist maps satisfying Milnor’s condition; in particular there are such maps for all n when k = 2. Those articles proved the existence of such maps but none of them exhibited explicit examples. An explicit example was constructed in 1973 by A’Campo for all n even and k = 2; this is given by the map from R2m+4 ∼ = Cm+2 into R2 ∼ = C, m ≥ 0, defined by 2 u + v¯) + z12 + · · · + zm . (u, v, z1 , . . . , zm ) −→ u v (¯

Thus an interesting problem is to find explicit families of real singularities satisfying the Milnor condition, and see which geometric properties they share with complex singularities. In fact Milnor’s fibration theorem for real singularities has another important difference with the complex case. In the theorem for complex 2n−1 − M ) −→ S1 is given singularities the projection map  φ of the fibre bundle (S by the obvious map φ = f |f |; for real singularities this is false in general, as Milnor pointed out in his book. To fix the ideas, we say that the function f satisfies the strong Milnor condition at 0 (a notation taken from [214]) if it further satisfies  that the projection map of the bundle (Sn+k−1 − M ) −→ Sk−1 is φ = f |f | . Similarly to complex singularities, a real singularity that satisfies the strong Milnor condition defines an open book decomposition of the sphere Sn+k−1 and its link is a fibred link. In [111] Jacquemard considered real analytic maps Rn+2 → R2 satisfying the Milnor condition, and he gave necessary conditions for such maps to satisfy the strong Milnor condition. It was shown recently in [218] that this is also related with the C-regularity studied by Bekka and others. Chapter VI lays down the main ideas that we use in Chapter VII to construct and study infinite families of singularities satisfying the strong Milnor condition, and which have a rich geometry. Chapter VII is based in [88, 226, 227, 214], but it is largely inspired by the pioneering work of L´ opez de Medrano, Verjovsky and Meersseman (LVM for short) about new constructions of complex manifolds via holomorphic dynamics, and by the previous work of Camacho-Kuiper-Palis about the topology of linear vector fields in the complex domain. As the previous comments suggest, the motivation for the constructions in these chapters comes from an entirely different setting to the one envisaged so far in this monograph. In fact, in [251] Ren´e Thom gave interesting ideas for the use of Morse theory to study foliations on smooth manifolds. In particular he noticed that given a foliation F on a smooth manifold M , and a Morse function f : M → R+ ∪ {0}, the restriction of f to the leaves of F has as critical set the

Introduction

7

critical points of f and the points where the level surfaces of f are tangent to the leaves of F . He showed that the latter is “generically” a manifold, except that it may possibly have certain singularities, and called it the variety of contacts (or polar variety) of the two foliations (F and the one given by the level surfaces of f ). Thom indicated how this polar variety could be used to study the foliation F . In the same vein, in [50] the authors look at linear vector fields F (z) = (λ1 z1 , . . . , λn zn ) , in Cn , where the λi are non-zero complex numbers, and look at the manifold V ∗ of points where the 1-dimensional holomorphic foliation F defined by F on Cn − 0 is tangent to the foliation S given by the spheres around the origin (i.e., by the level sets of the Morse function “distance to 0”, squared). It is an exercise to see that if 0 ∈ C is not contained in the convex hull H(λ1 , . . . , λn ) of the λi , then V ∗ is empty. In this case the vector field is said to be in the Poincar´e domain; in such a case all the leaves of F accumulate at the origin, being transversal to all spheres around 0. This situation is very important from the viewpoint of dynamical systems, but for this work the relevant case is when 0 is contained in H(λ1 , . . . , λn ), i.e., when F is a vector field in the Siegel domain. When this happens and the eigenvalues λi satisfy a certain genericity (the “weak hyperbolicity condition”), the polar variety V ∗ is actually a smooth manifold, defined by the complete intersection singularity, VΛ = { λ1 z1 z¯1 + · · · + λn zn z¯n = 0 } , which is singular only at 0, of real codimension 2, and V ∗ = VΛ − 0. The same constructions work if we consider several commuting linear vector fields (satisfying certain conditions). One obtains in this way real analytic complete intersection singularities of higher codimension, which have very rich and fascinating geometry and topology (see [149, 150, 151, 160, 268, 143, 161, 162, 31, 32], and also Chapter VI below). Alas, these singularities do not satisfy the Milnor condition at 0. However, one may replace the linear vector field (λ1 z1 , . . . , λn zn ) by any non-linear vector field of the form: F (z) = (λ1 zσa11 , . . . , λn zσann ) , where {σ1 , . . . , σn } is any permutation of the set {1, . . . , n}, ai ≥ 2 for all i = 1, . . . , n and the λi are arbitrary non-zero complex numbers; the corresponding polar variety VF∗ is again a smooth manifold, defined by the complete intersection VF = { λ1 zσa11 z¯1 + · · · + λn zσann z¯n = 0 } , VF∗ = VF − 0, which is singular only at 0, quasi-homogeneous, of real codimension 2. The remarkable fact is that these singularities do satisfy the strong Milnor condition at 0. Thus one has open book decompositions on the spheres analogous, but different, to those given by complex singularities. Notice that these singularities are reminiscent of the classical Pham-Brieskorn singularities λ1 z1a1 + · · · + λn znan = 0 ,

8

Introduction

and if the permutation σ is the identity, then they turn out to be topologically (not analytically) equivalent to Pham-Brieskorn singularities, so we call them twisted Pham-Brieskorn singularities (see Chapter VII). As an example, our results in Chapter VII show that the famous Poincar´e’s homology 3-sphere Σ can be regarded as the set of points in the unit sphere S5 ⊂ C3 where S5 is tangent to the holomorphic foliation spanned by the vector field F = (z13 , z24 , z36 ); the points where F is tangent to S5 as a real vector field form the double of the E8 -manifold, containing Σ as an equator. Along the way, in Chapter VI we look at the real hypersurfaces that arise when we consider a C-valued holomorphic function f on an open set U in Cn and we compose it with the projection onto a real line through the origin in C. We prove in particular that if f has an isolated critical point at 0, then the double of its Milnor fibre is diffeomorphic to the link of its real part. As a byproduct we get explicit real analytic embeddings in S3 of closed oriented 2-manifolds of all genera. Chapter VIII is in part a return to the most basic situation: knots and links defined algebraically in the 3-sphere. The goal here is to present the results of [202] about the singularities z1p z 2 + z2q z 1 = 0 ,

(∗)

which are of the class considered in the previous chapter, so we know they define open book decompositions of the 3-sphere “a la Milnor”. This situation is very much reminiscent of the classical problem, dating back to Newton and others, of studying the topology of plane curves, i.e., sets in C2 defined by f (z1 , z2 ) = 0, where f is a holomorphic function. One of the most classical ways for approaching this problem is via resolutions of the singularities. In fact, Max Noether (1883) proved that the singularities of every plane curve could be resolved by blowing ups; this means that given the germ at 0 of a curve C ⊂ C2 with a singular point  with a projection π : X  → C2 at 0, one can find a smooth complex surface X which is composition of a finite number of blow-ups, such that the divisor E = π −1 (0) is connected, consists of a finite number E1 , . . . , Er of copies of CP1 whose intersections are either empty or at ordinary double points (i.e., defined locally by  in X  of π −1 (C − 0) xy = 0), π is biholomorphic away from E, and the closure C (which is called the strict transform of C) consists of a finite number of pairwise disjoint complex lines, as many as branches of C, which intersect E transversally at smooth points of E. It is customary to assign to such a resolution a decorated plumbing graph, also called the resolution graph, which describes its topology. Since the topology of C (as an embedded sub-variety) is determined by the topology of C ∩ S3ε , where S3ε is a small sphere around 0 ∈ C2 , and π is a diffeomorphism away from E, it follows that if we know the resolution graph of the singularity, then we know the topology of the plane curve. This is what we aimed to do with the singularities (∗); the additional problem we face is that these are only real analytic and the usual process of resolutions via blow-ups gets stuck after the first step (i.e., the singularities are no longer simplified by the blow-ups). The trick here is to modify some of its branches

Introduction

9

by a homeomorphism in the second step of the resolution process, in order to make them become complex analytic singularities, which one can resolve in the usual way. In this way we get a “topological” resolution π : X → C2 of the singularity {z1p z 2 +z2q z 1 = 0 }, which determines the resolution graph and therefore the topology of these singularities. However, just as for complex curves, this is not enough to determine the topology of the corresponding Milnor fibrations. One has to do more: the links that we get in this way are all Seifert links in the sense of [75], and they are fibred, defining open book decompositions of the 3-sphere which are horizontal, i.e., the pages are transversal to the Seifert fibres. Pichon in [200] studied these type of links; more generally she studied Waldhausen links which are horizontally fibred, and found a way for describing fully the topology of these fibrations, using the resolution graph and the orientations on the components of the link induced by the open book decomposition. In the particular case of Seifert links, which is simpler, one has that the monodromy of the corresponding fibration is cyclic and one can associate to it a Nielsen graph, which encodes all the information of the resolution graph, and which is also determined by the Seifert (or the resolution) graph. One has that the Nielsen graph of the monodromy determines the period of it and the topology of the fibres (using the Hurwitz formula). Thus we have the complete topological description of these fibrations. The links of the singularities that we envisage in this chapter turn out to be isotopic to the links defined by the equations: z¯1 z¯2 (z1p+1 + z2q+1 ) = 0 , which are a special type of singularities of the form f g¯ with f, g holomorphic functions C2 → C. This type of singularities are already present in the work of N. A’Campo [2] and L. Rudolph [212]. In [199] Pichon proves that such a function f g¯ : R4 → R2 has an isolated critical point (say at 0) iff the link Lf ∪ Lg is fibred (as an oriented link), where Lf , Lg are the links of f and g. She further proved that in this situation the projection map: φ : Sε − (Lf ∪ Lg ) → S 1 can be taken to be |ff gg¯¯| in a tubular neighborhood of the link. These results have been recently improved in [201], relaxing the condition of f g¯ having an isolated critical point and showing that the projection φ can be taken as |ff gg¯¯| everywhere. We briefly explain this in the last section of Chapter VIII.

Acknowledgments This monograph is the outcome of many years of working in the area of singularities, during which I have profited from the work of many great mathematicians, to whom I am deeply indebted. Their works have been to me the lights in my path

10

Introduction

as a mathematician, and have shown me the beauty of mathematics. My thanks to all of them. I would like to mention here two people who have had the strongest impact and influence on me during all this time, and whose work permeates all areas of mathematics that I have explored: Sir Michael F. Atiyah and Professor John W. Milnor. During this time I have also profited from conversations with many people and I have had the support of several institutions which have either hosted me or given me financial support, or both. I am glad to have the opportunity of expressing here my gratitude to all of them. Although I am sure to leave out some important names, I want to mention those that I am able to remember. First of all, I am most grateful to Nigel Hitchin, Lˆe D˜ ung Tr´ ang and Alberto Verjovsky, for explaining to me many of the ideas and concepts included in this text, and for teaching me about the beauty of mathematics; talking to them has always been a source of inspiration for me. I have also profited a lot from conversations with a number of friends and colleagues, who have taught me a lot, especially: Jean Paul Brasselet, H´el`ene Esnault, Francisco Gonz´alez Acu˜ na, Santiago L´ opez de Medrano, David Massey, A.J. Parameswaran, Anne Pichon, Elmer Rees, Maria Aparecida Ruas, V. Srinivas, Brian F. Steer, Tatsuo Suwa, Eckart Viehweg, and surely others whose name slips away from my memory. My heart-felt thanks to all of them. I am very grateful to Juan Pablo Romero for helping me with the many intricacies of Latex and, mostly, for making the figures in this text. And to Elsa Puente for proof-reading the text and pointing out several misprints and corrections to be made. I am indebted to my wife Teresa for her patience and for being the driving force that moves me forward. Among the institutions that have hosted me and/or supported my work on the material of this text, I am most grateful to the Abdus Salam International Centre for Theoretical Physics at Trieste, Italy, for providing me the perfect atmosphere for concentrating on this work; I wrote the bulk of it while a Staff Associate of the ICTP. I am also very much indebted to the Institut de Math´ematiques de Luminy (France), to the Mathematical Institute of the University of Oxford (U.K.), to Instituto de Matem´ atica Pura e Aplicada de Rio de Janeiro (Brazil), University of Hokkaido (Japan) and Tata Institute of Fundamental Research (India). During this time I have had several grants from CONACYT (M´exico) and from the DGAPA of the National University of Mexico (UNAM), as well as from CNRS (France), SRC (U.K.), CNPq (Brazil) and from various other sources, and I am most grateful to all of them for this important support. And, of course, I am most grateful and indebted to the Instituto de Matem´aticas of UNAM and its Cuernavaca Unit, where I work, for its great support and stimulating atmosphere.

Chapter I

A Fast Trip Through the Classical Theory In this chapter we review briefly some of the classical results of Milnor and others about the topology of real and complex analytic spaces around an isolated singular point. We review first the theorem of Milnor about the (embedded) local conical structure of analytic spaces; then we recall Ehresman’s fibration theorem, the definition of open book decompositions; the fibration theorem of Milnor for real singularities in general and his improved version for complex singularities. Then we speak about the topology of the Milnor fibre, beginning with the case of the Pham-Brieskorn polynomials where the topology is given by the so-called “join of Pham”. Finally we speak briefly about the work of Brieskorn, Hirzebruch and Milnor on exotic spheres and singularities.

I.1

An example: the Pham-Brieskorn polynomials

Let us begin by considering, as an example, the Pham-Brieskorn polynomials in Cn+1 : f (z) = zoa0 + · · · + znan , where z = (zo , . . . , zn ) and the ai are integers ≥ 2, c.f. [40, 39, 198]. The derivative of f at a point z is: Df (z) = (a0 zoa0 −1 , . . . , an znan −1 ) . Hence the origin 0 is the only critical point of f and therefore the variety V = f −1 (0) = {zoa0 + · · · + znan = 0} is a hypersurface (complex codimension 1) with an isolated singularity at 0 ∈ Cn+1 .

12

Chapter I. The Classical Theory

In order to study the topology of V as an embedded subvariety of Cn+1 , let d be the least common multiple of the ai , i = 0, 1, . . . , n, set qi = d/ai and consider the action of the non-zero complex numbers C∗ on Cn+1 given by: t · (zo , . . . , zn ) = (tq0 zo , . . . , tqn zn ) . It is clear that if we restrict this action to R+ we obtain a real analytic flow in C satisfying: (i) its flow lines (the orbits) are transversal to all the spheres around 0 ∈ Cn+1 ; (ii) all the orbits accumulate at the origin as t tends to 0; and (iii) the variety V is an invariant set of the flow, i.e., V is a union of orbits. Notice this flow defines a 1-parameter group of diffeomorphisms {φt } of Cn+1 that has 0 as a fixed point, V as an invariant set and it is strictly contracting for t < 1, while it is expanding for t > 1. This implies that: (i) V intersects transversally every (2n+1)-sphere Sr (0) around the origin; hence the intersection Kr = V ∩Sr (0) is a smooth manifold of real dimension 2n− 1 embedded as a codimension 2 submanifold of the sphere Sr (0); (ii) the diffeomorphism type of Kr is independent of the choice of the sphere Sr (0); and (iii) the embedded topological type of V in Cn+1 is determined by the pair (Sr (0), Kr ), i.e., the pair (Cn+1 , V ) is homeomorphic to the (global) cone over the pair (Sr (0), Kr ). The manifold K = Kr , for some r > 0, is called the link of the singularity, and it is known as the Brieskorn manifold M(a0 ,...,an ) . Notice that we also have an S1 -action on Cn+1 obtained by restricting the above C∗ -action to the unit complex numbers. This S1 -action on Cn+1 is by isometries and therefore induces a locally free action on the links Kr . If n = 1 this means K = Kr , for some r > 0, is the union of a finite number of S1 -orbits (as many as the irreducible components, or branches, of V ). If n = 2 then K is a 3-manifold in S5 with an S1 -action, so K is a Seifert manifold, whose Seifert invariants can be easily determined from the “weights” of the action, see for instance [194]. In this case the corresponding Brieskorn manifolds have very interesting topology and this will be explained later in the text. For instance the link of the polynomial z1p + z2q + z3r is the lens space S3 /Zr when p = q = 2, and it is Poincar´e’s homology sphere for (p, q, r) = (2, 3, 5). The topology of these manifolds for n > 2 has been studied by Brieskorn [40, 39], Milnor [171] and others, obtaining remarkable results (see Section 8 below). As we mentioned above, for n = 1 the link is the union of a finite number of copies of the circle S1 , so its topology is rather simple. Still, we can look at the embedded topological type of K in S3 , which is a knot if it is connected, or a link in general (i.e., a disjoint union of knots). What type of knot or link is it? To answer this, assume for simplicity that p = 2 and q = 3 (essentially the same arguments hold whenever p, q are relatively prime). Notice that, up to isotopy, K n+1

I.1. Pham-Brieskorn polynomials

13

is defined by {z12 = z23 } and {|z1 |2 + |z2 |2 = 2}. Hence K is contained in the torus T = {|z1 | = |z2 | = 1} as the set of points given by: 2 arg z1 = 3 arg z2 . it

it

Thus we can parametrize K by taking a point t ∈ R into the point (e 2 , e 3 ) ∈ K, with t ∈ [0, 12π]. As we do so, the first coordinate wraps around the unit circle 3 times, while the second coordinate wraps around the circle 2 times. This means that the torus knot K goes around a parallel of T twice and it goes around a meridian of T thrice. The same arguments show that if p, q are relatively prime, then K is always a torus knot of type (p, q), i.e., it is contained in a torus in S3 in such a way that it goes around a meridian q times and around a parallel p times. For this we may use the parameterization of K given by γ(t) = (eit/p , eit/q ), which is periodic of period pq. More generally, if p = sp , q = sq  with p , q  relatively prime, then it is an exercise to show that K consists of s disjoint copies of a torus knot of type (p , q  ), so it is called a torus link of type (p, q). For instance, if the polynomial f is homogeneous of degree p, i.e., p = q > 1, then K consists of p fibres of the Hopf fibration S3 → S2 , which are circles embedded in T of type (1, 1), since they are given by the orbits of the standard S1 -action on C2 defined by eit · (z1 , z2 ) = (eit z1 , eit z2 ).

Figure 1: Toral knots of types (2, 5) and (3, 4).

1.1 Remarks on the topology of plane curves and curves in surfaces. The literature about this subject is vast, going back to work by Newton, Max Noether and others; Brauner [38] was the first one to study the topology of plane curves C in C2 (i.e., the zero set of a complex analytic map C2 → C with an isolated critical point at 0 ∈ C2 ) by looking at the intersection of C with a small sphere Sε around 0; this intersection defines a knot (or link) K in the 3-sphere, and the pair (Sε , K)

14

Chapter I. The Classical Theory

determines the local topology of C near 0. The book of Milnor gives an account of this construction and more (see Chapter 10 in [168]), and this should become clear later in the text. The book [44] of Brieskorn and Kn¨ orrer contains everything we may pretend to say here about knots defined by complex plane curves, so we refer to that excellent text for those wanting to dive into this fascinating subject. This is closely related with the content of Chapter VIII below, so we shall return to plane curves in that chapter. We also refer to [249], where Teissier gives a very clear introduction to the subject. For a more advanced reading see the book [75] of Eisenbud and Neumann which describes important constructions of “complicated” links from simple pieces, and how to use graphs to study and describe this process. See also [128] for a short and clear presentation of the classic results on the topology of complex analytic plane curves from the differentiable viewpoint. The more recent article [134] studies the three-dimensional manifold M given by the complement in S3ε of an algebraic link K, i.e., a link in the 3sphere defined by a complex plane curve. See also [200], which is concerned with knots in 3-dimensional manifolds which appear as links of complex normal surface singularities (see §2 below).

I.2

The local conical structure

We study here a construction that goes back to Brauner in [38] and was sketched in Section 1 above. Consider a (reduced, equidimensional) real analytic space V of dimension n, defined by a finite number of real analytic equations in an open ball Br (0) ⊂ RN around the origin, and assume further that V contains the origin 0 and V ∗ := V − {0} is non-singular, i.e., it is a real analytic manifold of dimension n > 0. We shall see that much of what we said in the previous section still holds in this more general setting provided we are sufficiently near the origin. Consider the function d : RN → R given by d(x1 , . . . , xN ) = x21 + · · · + x2N , so that d is the square of the function “distance to 0”. Its gradient vector field ∇d is given by (2x1 , . . . , 2xN ), so its solutions are the straight rays that emanate from the origin. Let us modify this vector field a little so that it “takes V into account”. In order to make the above statement precise, take the restriction dV of d to V . At each point x ∈ V ∗ the gradient ∇dV (x) of dV is obtained by projecting ∇d(x) to Tx V ∗ , the tangent space of V ∗ at x. Since ∇d(x) is never zero on V ∗ and it is a radial vector transversal to the sphere Sx passing through x and centred at 0, it follows that ∇dV (x) vanishes if and only if Tx V ∗ ⊂ T Sx . This means that the critical points of dV , other than 0 itself, are the points in V ∗ where this manifold fails to be transversal to the sphere passing through the given point. In other words, a point x ∈ V ∗ is a critical point of dV iff V ∗ is tangent at x to the sphere passing through x and centred at 0. Just as in [168, Corollary 2.8], one has that dV has at most a finite number of critical values corresponding to points in V ∗ , since it is the restriction of an analytic function on Br (0). Hence for

I.2. The local conical structure

15

V ∩ S2N −1

Figure 2: The conical structure

ε > 0 sufficiently small the function dV has no critical points on V ∩ Bε ; therefore V ∗ meets transversally all sufficiently small spheres around the origin in RN . The gradient vector field of dV is now everywhere transversal to the spheres around 0, and it can be assumed to be integrable. Hence it defines a 1-parameter family of local diffeomorphisms of V ∗ taking each link into “smaller” links. Thus, near 0, V is the cone over the intersection V ∩ Sε of V with a small sphere around the origin and the diffeomorphism type of the intersection V ∩ Sε is independent of ε for all sufficiently small spheres. A more refined argument, due to A. Durfee [66] and based on the “Curve Selection Lemma” of [168], shows that the diffeomorphism type of the manifold K := V ∩ Sε is also independent of the choice of the embedding of V in RN . In other words, up to diffeomorphism K depends only on the analytic structure of V at 0 and not on the choice of the equations that define V nor on the sphere Sε , provided this is small enough. This same result is also proved by Lˆe-Teissier in [136].

16

Chapter I. The Classical Theory

Figure 3: The link of the singularity determines the topological type.

2.1 Definition. The manifold K := V ∩ Sε is called the link of V at 0. It is worth saying that the word “link” is being used in two different ways. In Definition 2.1 the name “link” comes from the notation used in PL-topology to denote the boundary of a regular neighborhood of a point in a triangulated manifold. But we also used the word “link” in Section 1 above to denote a disjoint union of circles in the 3-sphere. Both names are common in the literature. Thus given a polynomial map in two complex variables, as for instance f (z1 , z2 ) = z1p + z2q , one has that its link (i.e., the intersection K = f −1 (0) ∩ Sε ) is a link in the 3-sphere Sε (i.e., a disjoint union of circles embedded in the sphere). This is why some authors prefer to call the manifold K of 2.1 in some other way, as for instance “the boundary of the singularity”. However we prefer to stick to the classical notation and call K the link of the singularity, just taking care that this does not lead to any confusion. Notice however that we have not said anything yet about the embedded topological type of V . We claim that just as for the Pham-Brieskorn singularities of the previous section, one has in general that for ε > 0 sufficiently small, the pair (Bε , V ∩ Bε ) is homeomorphic to the cone over the pair (Sε , V ∩ Sε ). In other words, not only is V ∩ Bε the cone over V ∩ Sε (as we know already), but the pair (Bε , V ∩ Bε ) is the cone over the pair (Sε , V ∩ Sε ). To show this we only need to refine a little the above argument, following [168]: for each x ∈ V ∗ , let Ux be an open neighborhood of x in Bε , and let rx be a vector field on Ux obtained by parallel extension of the gradient vector field ∇dV on Ux ∩ V ∗ . We may assume that the covering {Ux } of V ∗ is locally finite. Now take the radial vector field ∇d on Bε − V , and glue all these vector fields by a partition of unity to obtain a vector field on Bε − {0}, which is everywhere transversal to the spheres around 0 and is tangent to V ∗ . Furthermore, we can choose this vector field to be integrable

I.3. Ehresmann’s fibration lemma

17

(see [168]). One has that the solutions of this vector field converge to 0 and provide a local family of diffeomorphisms that yield the following theorem. 2.2 Theorem (Milnor). The homeomorphism type of V near 0 and the isotopy class of the embedding of V in RN are determined by the intersection of V with a small sphere Sε around the isolated singularity 0 ∈ V . More precisely, for every sufficiently small ball Bε around 0 one has a homeomorphism of pairs: (Bε , Bε ∩ V ) ∼ = Cone (Sε , Sε ∩ V ) , where Sε = ∂Bε is the boundary. 2.3 Remark. It is well known that Theorem 2.2 holds for non-isolated singularities. The main difference is that in this situation the link K will no longer be a manifold but a singular space. Actually, given an analytic germ (V, p), if we only care for the topology of V near p, then the fact that V is locally a cone over the link of p follows immediately from the work of Lojasiewicz in [144], where he proved that every analytic space can be triangulated. But one can say more, just as in [168]: given an embedding of (V, p) in some affine space RN , the embedded topological type of V is a cone over the intersection of V with a small sphere around p. The proof of this result is along the same lines as that of Theorem 2.2 above; this can be found for instance in [99, 48]. The proof uses Whitney stratifications (see for instance [136]). The idea of the proof is that given a real analytic germ (V, 0), we can choose the representative of (V, 0) small enough so that there exists a Whitney stratification {Sα }α∈A of the ball Bε with a finite number of strata, compatible with V (i.e., V is the union of strata), and such that each stratum meets transversally all the spheres in Bε with centre at 0. Each stratum Sα is a real analytic submanifold of Bε , so one can construct a vector field on Sα just as we did for V ∗ above, by considering the gradient vector field of the restriction to Sα of the function distance to 0 (squared). The Whitney conditions guarantee that we can put all these vector fields together, by a partition of unity, in order to obtain a vector field on all of Bε (for ε > 0 sufficiently small), which is transversal to all small spheres around the origin, pointing outwards, and it is stratified, i.e., at each point x ∈ V it is tangent to the stratum Sα that contains x. In this way we obtain a 1-parameter family of homeomorphisms taking the pair (Sε , Sε ∩ V ) into the “smaller” links (Sε
, Sε
∩ V ) for ε > ε > 0, and these homeomorphisms preserve each stratum Sα .

I.3

Ehresmann’s fibration lemma

The content of this section is for smooth mappings between manifolds and is included in this text for completeness, since it motivates the fibration theorem for real singularities of Milnor that we shall prove in the next section. This result was originally proved in [74] (see also [1], for instance).

18

Chapter I. The Classical Theory

3.1 Theorem (Ehresmann’s fibration lemma). Let M and N be (C∞ for simplicity) oriented, differentiable manifolds of dimensions n + k and k respectively. Assume M is closed (=compact without boundary) and let f :M →N be a proper differentiable  which is a surjection everywhere, i.e., the Jacobian  ∂fi map (x) has rank k for each x ∈ M . Then f is the projection matrix Df (x) = ∂x j map of a (locally trivial) fibre bundle. Proof. The fact that f is a submersion everywhere implies that all points in M are regular points of f , so the Implicit Function Theorem implies that all the fibres f −1 (y) , y ∈ N , are smooth submanifolds of M of codimension k, i.e., dimension n. So the proof of Theorem 3.1 amounts to proving the local triviality of the projection. In other words we must show that given any y ∈ N there exists an open disc Dy ⊂ N such that f −1 (Dy ) is a product Dy × f −1 (y). At each point x ∈ M we have a splitting of the tangent bundle of M as Tx M ∼ = Tx (f −1 (y)) ⊕ νx (f −1 (y)) , y = f (x) , where ν(f −1 (y)) is the normal bundle of the fibre of f containing x, for some Riemannian metric. The fact that Df (x) has rank n implies that νx (f −1 (y)) is carried by Df (x) isomorphically into the tangent bundle of N at y. Therefore the normal bundle of f −1 (y) is trivial and it is isomorphic to the pull-back by f of the tangent space Ty N . Let {a1 , . . . , ak } be a basis of Ty N . We can always extend this to a set of k linearly independent integrable vector fields on a small disc Dy of y in N , which we still denote by {a1 , . . . , ak }. These define k local flows around y which can be used to parametrize Dy , since the vector fields {a1 , . . . , ak } are linearly independent. By the arguments above one has that {a1 , . . . , ak } lift to a trivialization {α1 , . . . , αk } of the normal bundle ν(f −1 (y)), which extends to a set of n integrable vector fields on a tubular neighborhood Ny of the fibre f −1 (y), linearly independent everywhere and orthogonal to all the fibres of f . By the compactness of the fibres of f , there exists a time t > 0 such that all these flows on Ny are defined for all times t with |t | < t and they do not have fixed points. Moreover, we can choose the flows so that they parametrize the neighborhood Ny . By re-scaling the time, if necessary, we can make this parametrization of Ny compatible with the one on the base Dy . Hence the neighborhood Ny is of the form Dy × f −1 (y), proving Theorem 3.1.


I.4

Milnor’s fibration theorem for real singularities

We now consider a real analytic map, f = (f1 , . . . , fk ) : (Br ⊂ Rn+k , 0) −→ (Rk , 0) , where Br is a (sufficiently) small ball around 0 in Rn+k .

I.4. Milnor’s fibration for real singularities

19

4.1 Definition. We say that f satisfies the Milnor condition at 0 ∈ Rn+k if its Jacobian matrix Df (x) has rank k for all x = 0 in Br . In other words this means that f is a surjection at every point in Br , except (possibly) at the origin. As Milnor himself points out in his book, this is a very stringent hypothesis when k > 1. We shall discuss this point later in the text, in Chapter VII. Notice that if f satisfies the Milnor condition at 0, then by the first ThomMather transversality theorem (see [1]), if V = f −1 (0) is transversal to a given sphere Sε around 0, then the fibres f −1 (t) will also be transversal to Sε for all t ∈ Rk sufficiently near 0. The following result of Milnor is a straightforward generalization of Ehresmann’s fibration lemma, and its proof is just a refinement of the proof of Theorem 3.1 so we only sketch it. For Milnor this was an intermediate step for his fibration theorem for real singularities, which we state below as Theorem 4.3. However we state it as a theorem because this is the version of Milnor’s fibration theorem which is more common in the literature and it also generalizes more naturally to other settings (c.f. the last section in this chapter and Chapter VII below). 4.2 Theorem. Let f : (Br ⊂ Rn+k , 0) −→ (Rk , 0) be real analytic and assume it satisfies the Milnor condition at 0. Let ε > 0 be sufficiently small so that Sε and all the spheres around 0 in Rn+k of radius less than ε meet V = f −1 (0) transversally. Let δ > 0 be small enough with respect to ε so that all fibres f −1 (t) with t ≤ δ be the sphere in Rk of radius δ and centre at 0. meet Sε transversally. Let Sk−1 δ Then the map ) ∩ Bε −→ Sk−1 , f : f −1 (Sk−1 δ δ where Bε ⊂ Rn+k is the (open or closed) ball bounded by Sε , is the projection of a (locally trivial) fibre bundle. Proof. By hypothesis the derivative of f is surjective at each point away from 0. For x ∈ V ∗ = V − {0} the kernel of Df spans the tangent bundle of the fibres of f while the pull-back of Tt Dδ spans the normal bundle ν(f −1 (t)) of f −1 (t), where . Hence a choice of a basis for Tt Dδ lifts to a Dδ is the disc in Rk bounded by Sk−1 δ trivialization of ν(f −1 (t)). Furthermore, since the fibres of f are transversal to the boundary sphere Sε , the sections {s1 , . . . , sk } that define this trivialization can be chosen to be tangent to Sε . Each section si generates a local flow ψλi , λ ∈ R, and by compactness we can choose a time t0 for which all these flows are well defined on all the fibres of f . This implies that each fibre of f has a neighborhood which is a product, proving Theorem 4.2.
It is worth saying that Theorem 4.2 could have been stated (with the same proof) as giving a fibre bundle over all of Dδ − {0}. Let us set N (ε, δ) := f −1 (Dδ )∩ Bε with Bε being, from now on, the closed ball. This manifold N (ε, δ) is usually referred to as a Milnor tube for f . The boundary of N (ε, δ) is the union of the

20

Chapter I. The Classical Theory

manifold f −1 (Sk−1 ) ∩ Bε in Theorem 4.2, together with the manifold δ Tε,δ := N (ε, δ) ∩ Sε , which is a regular neighborhood of K = V ∩ Sε in the boundary sphere Sε . 4.3 Theorem. Let f be as in Theorem 4.2. Then, with the above hypothesis and notation, the fibre bundle f : N (ε, δ) − V −→ Dδ − {0} induces a fibre bundle projection: = ∂Dδ φ : Sε − Int (Tε,δ ) −→ Sk−1 δ which, restricted to the boundary ∂Tε,δ = f −1 (Sk−1 ) ∩ Sε , is the map f . δ This theorem is a consequence of Theorem 4.2 and the following lemma (which is Theorem 1 in [167]). 4.4 Lemma. The boundary of the tube N (ε, δ) is homeomorphic to Sε under a homeomorphism that leaves Tε,δ pointwise fixed. In fact the same proof shows that N (ε, δ) is homeomorphic to the ball Bε . Proof. We follow the arguments of Milnor in [167], just replacing the hard part in his proof by Corollary 3.4 in [168], which is a consequence of the Curve Selection Lemma. Let f1 , . . . , fk be the components of f and define a function: r(x) = f12 (x) + · · · + fk2 (x) . Let ∇r be its gradient. The level surfaces r−1 (s), s ∈ R+ are the tubes f −1 (|t|) for |t|2 = s, and the vector field ∇r is transversal to these tubes (away from V ). Consider also the vector field ∇(ι)(x) = 2x, which is the restriction to Dε − V ι of the gradient of the function x → x2 . Both maps r and ι are polynomial and ≥ 0. Hence Corollary 3.4 of [168] implies that on Bε − V the vector fields ∇r and ∇(ι) are either linearly independent or one is a positive multiple of the other. Now define a vector field on Bε − V by: v(x) = x · ∇r + ∇r · x , which bisects the angle between ∇r(x) and x. This is a smooth vector field on Bε − V and by the Schwartz inequality (v1 v2  ≥ v1 · v2 , with equality holding only when the two vectors are colinear) it satisfies: v·x>0 ,

v · ∇r > 0 .

(∗)

Now, given any point xo ∈ Bε − V , let γ(t) be the solution through xo of the differential equation dx/dt = v(x) in Bε − V . As we move along the path γ(t),

I.5. Open books and fibred knots

21

by (∗) the distance to 0 increases strictly, and so does the function r, until γ(t) intersects the boundary sphere. We may thus define a homeomorphism h : ∂N (ε, δ) − Int(Tε,δ ) → Sε − Int(Tε,δ ) as follows: given x ∈ ∂N (ε, δ) − Int(Tε,δ ), take the path γ(t) passing through x and follow it until it meets the sphere at a point γ(t1 ), then define h(x) = γ(t1 ). It is clear that this homeomorphism is the identity on the boundary of Tε,δ , so we can extend it to the interior as the identity.


I.5

Open book decompositions and fibred knots

The two concepts in the title of this section were introduced after Milnor published his book on singularities, so he does not mention these at all. However these important concepts are very useful in many ways, so I include them here because they help us to grasp better what Milnor’s theorem for complex singularities is telling us, and what is the difference with the general statement in the real case. The concept of “open book decompositions” was introduced by E. Winkelnkemper in [264] and we refer to his appendix in [205] for a clear and updated account on the subject (see also [210]). In [247] Tamura defines the equivalent notion of “spinnable structures” on manifolds. Here we only give the basic notions that we use in the sequel. 5.1 Definition. An open book decomposition of a smooth n-manifold M consists of a codimension 2 submanifold N , called the binding, embedded in M with trivial normal bundle, together with a fibre bundle decomposition of its complement: π : M − N → S1 , satisfying that on a tubular neighborhood of N , diffeomorphic to N × D2 , the restriction of π to N × (D2 − {0}) is the map (x, y) → y/y. The fibres of π are called the pages of the open book. It follows from the definition that the pages are all diffeomorphic and each page F can be compactified by attaching the binding N as its boundary, thus getting a compact manifold with non-empty boundary. Also, since the base of the fibration is the circle S1 , one can lift a never-zero vector field on S1 to an integrable vector field on M − N which is transversal to the fibres. Using the flow lines of this vector field one can define a “first return map” on the fibres, which is well defined up to isotopy. This diffeomorphism is known as the monodromy of the fibration. Since all the pages have the same binding N as boundary, it follows that h extends as the identity on N . This brings us back to the original definition of open books in [264]. Winkelnkemper defined open books as follows: start with a compact manifold F with nonempty boundary ∂F , together with a diffeomorphism h of F which is the identity on ∂F . Now form the mapping cylinder F h of h, which is a manifold with boundary

22

Chapter I. The Classical Theory

F N Fπ /2 Fπ /4

N F0

Figure 4: An open book.

(∂F ) × S1 . Identifying (x, t) with (x, t ) for each x ∈ ∂F and t, t ∈ S1 , we obtain a closed, differentiable manifold M . The fibres F × t are the pages of the open book, and their common boundary N = ∂F × t is the binding. Notice that in this definition the pages are already compact manifolds with boundary, and their interiors are the pages in Definition 5.1. The concept of “fibred knots and links” actually arises from Milnor’s fibration theorem for complex singularities. This was introduced by A. Durfee and B. Lawson in a paper [67] that generalizes the beautiful construction by Lawson, using Milnor’s theorem, of codimension 1 foliations on odd-dimensional spheres. We recall that a classical knot is an embedded circle in S3 , and a classical link is a disjoint union of knots in the 3-sphere. These notions have been generalized as follows. 5.2 Definition. Let M be a smooth, closed, connected manifold. A knot in M means a smooth codimension 2 closed, connected submanifold N of M . If N has several connected components then it is called a link in M . 5.3 Definition. A knot (or link) N ⊂ M is fibred if it is the binding of an open book decomposition of M .

I.6

On Milnor’s fibration theorem for complex singularities

Consider now a complex analytic map, f : (Br ⊂ Cn+1 , 0) −→ (C, 0) with a critical point at 0 ∈ Cn+1 , which may be a non-isolated critical point of f . One has (Milnor’s fibration theorem for complex singularities):

I.6. Milnor’s fibration for complex singularities

23

6.1 Theorem. Let ε > 0 be sufficiently small and set V = f −1 (0) and K = V ∩ Sε . Then the map φ : Sε − K → S1 given by φ(z) =

f (z) |f (z)|

is the projection map of a (locally trivial) fibred bundle. Furthermore, if V has an isolated singularity at 0, then K ⊂ Sε is a fibred link, defining an open book decomposition of the sphere Sε ∼ = S2n+1 . In fact Milnor also showed that whenever n > 1, the link K is connected, so it is a knot in the sphere. [Milnor actually proved that K is (n − 2)-connected, i.e., its homotopy groups vanish up to dimension n − 2.] 6.2 Definition. A knot (or link if n = 1) defined by a holomorphic function as in Theorem 6.1 is called an algebraic knot (or link). The notion of “algebraic knot” was introduced by Lˆe D˜ ung Tr´ ang in [125], and we refer to [127, 128, 62, 63] for basics about these knots. The proof of Theorem 6.1 is written in a very clear and elegant way in Milnor’s book, so I will content myself by making a few comments about it. It is worth saying that Milnor’s proof was written for polynomial maps, but all his arguments go through for holomorphic maps in general. We also refer the reader to [201] where Milnor’s proof is adapted to meromorphic functions f /g and the key ideas are carefully explained. The point is to show first that the map φ in Theorem 6.1 has no critical points and so its fibres are all codimension 1 smooth submanifolds of the sphere Sε , and then construct a tangent vector field on Sε − K which is transversal to the fibres of φ and whose solutions move at constant speed with respect to the argument of the complex number φ(z); this shows the local product structure around each fibre. It is of course natural to compare Theorem 6.1 with the corresponding theorem for real singularities. In fact one is tempted to try to prove Theorem 6.1 by “pushing Theorem 4.3 further”, and I believe this was the original approach followed by Milnor (see [167]). Two first difficulties, which are not serious problems, are that Theorem 4.3 requires an isolated critical point of f and it only gives a fibration away from a tubular neighborhood of the link. As pointed out by Milnor in [167, Chapter 11], this latter difficulty can be easily overcome and the fibration in Theorem 4.3 can be extended to the whole complement of the link with some extra work. The first difficulty can mean a problem, because the statement in general is false for real maps with non-isolated critical points. In fact it is false (in general) even for holomorphic maps which define complete intersection germs with non-isolated singularities. This was explained to me by Lˆe D˜ ung Tr´ang, who gave me the following example, which arises from Whitney’s umbrella and is known as “Lˆe’s example” (unpublished). Consider the function f : C3 → C2 given by f (x, y, z) = (x2 − y 2 z, y). Its critical set Σ consists of the points where the Jacobian matrix Df has rank less than 2. One has:   2x −2yz y 2 Df (x, y, z) = . 0 1 0

24

Chapter I. The Classical Theory

Thus Σ consists of the z-axis {x = y = 0}. Notice Σ = f −1 (0), and there are no more critical points of f . However the fibre over the point (t2 , 0) t ∈ R consists of the parallel lines (±t, 0, z), while the fibre over a point of the form (t2 , λ) , λ = 0), is the quadric {x2 − λ2 z = t2 }. Hence there cannot possibly be a local Milnor fibration. However this problem cannot occur for the situation envisaged in Theorem 6.1, that is when f is holomorphic and the target is 1-dimensional, essentially because in this case one has a theorem due to Hironaka [100], saying that f satisfies the “Thom af -condition” (see [136]). We can then follow the technique of Lˆe D˜ ung Tr´ ang in [129] to show that one has a local Milnor fibration whenever one has the af -condition (see [201]). There is one more difficulty for proving Theorem 6.1 using Theorem 4.3, but this again is a technical difficulty that can be overcome, and this is essentially what Milnor does in Lemma 5.9 in [168]. Indeed Theorem 4.3 is proved by using the fibration of Theorem 4.2, which for the case envisaged in Theorem 6.1 takes the form: f : f −1 (S1δ ) ∩ Bε −→ S1δ , ) ∩ Bε by means of a vector field v(z) so and then “inflating” the tube f −1 (S2n+1 δ that the tube becomes the complement in S2n+1 of an open tubular neighborhood of the link K. In general one does not have much control of the behaviour of this vector field and therefore one cannot say anything in general about the map φ in Theorem 4.3 nor its relation with the original map f . Still, in the complex case Milnor manages to construct a vector field in his Lemma 5.9, that we denote here by vˆ, making the same job as the vector field v in the proof of Theorem 4.3, with the additional feature that the flow lines of vˆ carry the tube f −1 (Sδ2n+1 ) ∩ Bε into the sphere, in such a way that the argument of the complex number f (z) is constant along each flow line. Hence Milnor can assure that the fibres of the fibration given by Theorem 4.2 are taken into the fibres of Theorem 6.1. There is finally one more difficulty, and this is a serious one: the behavior near the link K. There are real analytic singularities given by maps R2n → R2 with a fibration as in Theorem 4.2 but which do not define an open book decomposition of the sphere S2n−1 (see [201]). Hence, in order to actually control the behavior of the fibres near the link, Milnor has to use completely different arguments (Section 4 of his book), which eventually are sufficient to complete the proof of Theorem 6.1 without using Theorem 4.2. Therefore the fibration Theorem 4.2 is given in [168] just as “an alternative” way to look at the fibration. Still, this alternative way has several advantages on its own and, as mentioned earlier, this is probably the most common version of Milnor’s theorem. 6.3 Remarks. Milnor’s fibration Theorem 4.3 was generalized by Hamm in [95] to germs of complex analytic, isolated complete intersection singularities; we refer to Looijenga’s book [146] for a clear account on this subject. One may also consider the germ of an arbitrary complex analytic variety (V, p) whose singular set may have dimension more than 0, and the main theorem of [129] says that in this

I.7. The join of Pham and the Milnor number

25

situation one has a topological locally trivial Milnor fibration as in Theorem 4.3 above. It is worth remarking that Milnor’s theorem also proves that one has a fibration as in Theorem 4.3 (essentially the same proof as above) even when the critical point of f is not isolated, and these two fibrations are equivalent. By the theorem of Andreotti-Frankel [6], this implies that the fibres of φ have the homotopy type of a CW-complex of middle dimension, and they are always parallelizable manifolds. (Milnor proves these facts directly, not using [6], because he needs essentially the same arguments to show that the link is highly connected.) These results have been generalized recently in [269, 201] for meromorphic functions: 6.4 Theorem. Let f, g : (Cn+1 , 0) → (C, 0) be holomorphic maps, so that all points in a punctured neighborhood of 0 ∈ C are regular points of the meromorphic map f /g : (Cn+1 , 0) → (C, 0) . Let Lf g = (f g)−1 (0) ∩ Sε be the link of f g. Then the map: φ :=

f /g fg = : Sε \ Lf g −→ S1 ⊂ C |f g| |f /g|

is the projection of a C∞ (locally trivial) fibre bundle, whose fibres Fθ are diffeo◦

morphic to the complex manifolds (f /g)−1 (t)∩ Dε , where t ∈ C is a regular value ◦

of the meromorphic map f /g and Dε is the interior of the disc in Cn+1 whose boundary is Sε . Hence each fibre is a parallelizable manifold with the homotopy type of a CW-complex of dimension n. We remark that this theorem is stated in [201] in terms of the function f g; as explained there, both formulations are essentially equivalent.

I.7

The join of Pham and the topology of the Milnor fibre. The Milnor number

In his pioneer article [198], F. Pham studied the topology of the complex manifold V(a0 ,...,an ) ⊂ Cn+1 defined by: z0a0 + · · · + znan = 1 , where n > 0 and the ai are integers ≥ 2. It is easy to see that this manifold is diffeomorphic to the Milnor fibre of the complex singularity defined by f (z) = 0, where f is the Pham-Brieskorn polynomial f (z) = z0a0 + · · · + znan . To explain Pham’s results, let Ga denote the finite cyclic group of ath roots of unity. Given the integers {a0 , . . . , an }, denote by J = J(a0 ,...,an ) the join: J = Ga0 ∗ Ga1 ∗ · · · ∗ Gan ⊂ Cn+1 ,

26

Chapter I. The Classical Theory

which consists (see [164]) of all linear combinations (t0 ω0 , . . . , tn ωn ) with the ti real numbers ≥ 0 such that t0 + · · · + tn = 1 and ωj ∈ Gaj . Note that J can be identified with the subset P = P(a0 ,...,an ) defined by: a

a

P = {z ∈ V(a0 ,...,an ) | zj j ∈ R and zj j ≥ 0 , for all j = 0, . . . , n} . To see this, notice that P can also be described by the conditions: zj = uj |zj | , uj ∈ Gaj , tj = |zj |aj , for all j = 0, . . . , n. Hence P is contained in the manifold V(a0 ,...,an ) . The set P is known as the join of Pham of the polynomial f . It is not hard to see that V(a0 ,...,an ) has P as a deformation retract and therefore its homotopy type is that of P. In fact, given a point z ∈ V(a0 ,...,an ) , first deform each coordinate zj along a path in C chosen a so that the trajectory described by zj j is the straight line to the nearest point on z0 , . . . , zˆn ) the real axis, that we denote by zˆj . This carries z into a vector zˆ = (ˆ a satisfying zˆj j ∈ R for each j, and it is clear that this deformation leaves V(a0 ,...,an ) a invariant. Now, whenever one has that zˆj j < 0, move zˆj along a straight line to 0 ∈ C, moving the other components accordingly so that the deformation leaves z0 , . . . , zˆn ) moves along a straight line V(a0 ,...,an ) invariant. Hence the point zˆ = (ˆ towards a point zˇ = (ˇ z0 , . . . , zˇn ) ∈ V(a0 ,...,an ) whose coordinates are all ≥ 0 and one has that each coordinate zˇj is necessarily of the form tj ωj for some tj ≥ 0 and some ωj ∈ Gaj . This gives a deformation of V(a0 ,...,an ) into P that leaves this set invariant, so the join P is a deformation retract of V(a0 ,...,an ) . It is now an exercise to show that P has the homotopy type of a wedge (or bouquet) of spheres of real dimension n. Moreover, the number of spheres in this wedge is (a0 − 1) · (a1 − 1) · · · (an − 1). Thus we have obtained: 7.1 Theorem (Pham). The variety V(a0 ,...,an ) := {z ∈ Cn+1 | z0a0 + · · · + znan = 1 } has the set P as a deformation retract. Thus V(a0 ,...,an ) has the homotopy type of a bouquet Sn of spheres of dimension n, the number of spheres in this wedge being [(a0 − 1) · (a1 − 1) · · · (an − 1)]. Let us now return to the situation envisaged in Section 6, of a complex analytic map f : (Br ⊂ Cn+1 , 0) −→ (C, 0) with an isolated critical point at 0 ∈ Cn+1 . One has Milnor’s fibration for f : f : f −1 (S1δ ) ∩ Bε −→ S1δ . It is easy to see that for the Pham-Brieskorn polynomials f (z) = z0a0 + · · · + znan one can take δ = 1 and Bε as the unit ball, so that the above manifold V(a0 ,...,an )

I.7. The join of Pham and the Milnor number

27

is the Milnor fibre F . By Theorem 7.1, in this case F has the homotopy type of a bouquet of spheres of dimension n. More generally one has: 7.2 Theorem (Milnor). Let f : (Br ⊂ Cn+1 , 0) −→ (C, 0) be complex analytic, with an isolated critical point at 0 ∈ Cn+1 . Then the fibre F = f −1 (t) ∩ Bε is a parallelizable manifold with the homotopy type of a bouquet Sn of spheres of middle dimension n. The number µ(f ) of spheres in this bouquet is ≥ 0 and it is 0 iff 0 ∈ Cn+1 is a regular point of f . Notice that the manifold F can be equivalently defined by F = φ−1 (eiθ ) for some eiθ ∈ S1 ⊂ C, where φ : Sε − K → S1 is given by φ(z) = f (z)/|f (z)|. The proof of Theorem 7.2 in [168] is by Morse theory and it has several steps. The first one shows that F has the homology of a CW-complex of dimension n. But if we think of F as being defined by f −1 (t) ∩ Bε , then this is an immediate consequence of the theorem of Andreotti-Frankel in [6], saying that the homology of every Stein space vanishes above the middle dimension. This fact implies, in particular, that F cannot have any compact component. Since it is a hypersurface in Cn+1 , its normal bundle is trivial and canonically trivialized by the gradient vector field ∂f ∂f ∇f (z) = ( ∂z , . . . , ∂z ). We recall that a smooth manifold is parallelizable if its 0 n tangent bundle is trivial, and a rather simple and elegant argument in [113] shows that a manifold with no compact component is parallelizable iff it can be embedded in some Euclidean space with trivial normal bundle. Hence F is parallelizable. 7.3 Definition. The manifold F is called the Milnor fibre of f at 0. The number µ = µ(f ) of spheres in the bouquet given by Theorem 7.2 is called the Milnor number of f at 0. It is worth saying that the Milnor number has several different interpretations, each pointing towards different properties of this invariant. The following theorem summarizes some of these properties, and it is all in [168]. 7.4 Theorem (Milnor). The number µ has the following interpretations: (i) it equals the number of vanishing cycles on F = f −1 (t) ∩ Bε ; (ii) it equals the multiplicity of the map-germ at 0 ∈ Cn+1 defined by the partial (f0 (z),...,fn (z)) derivatives of f . That is, µ is the degree of the map z → (f 0 (z),...,fn (z)) from a small sphere Sε centred at 0 into the unit sphere in Cn+1 , where ∂f fi = ∂z ; i (iii) it is the dimension of the vector space obtained by taking the quotient of the local ring O0 (Cn+1 ) of holomorphic functions at 0 ∈ Cn+1 by the Jacobian ideal of f : µ = dim O0 (Cn+1 )/((f0 (z), . . . , fn (z)) (iv) it is determined by the Euler characteristic of F : µ = (−1)n+1 (1 − χ(F )) . These are all immediate consequences of Theorem 7.2. In fact we know that near the singular point the variety V is locally a cone, so it has the homotopy of a

28

Chapter I. The Classical Theory

Figure 5: The Milnor fibration.

point and its homology vanishes in dimensions > 0. On the other hand Theorem 7.2 tells us that for each t = 0 with |t| sufficiently small, the homology of f −1 (t)∩Bε in the middle dimension is a free Abelian group in µ generators. As the parameter t tends to 0 the non-singular fibre degenerates into V ∩ Bε and the homology of F disappears. To be precise, the ball Bε has V ∩ Bε as a deformation retract (see the proof of Lemma 4.4 above); let r be such a retraction and let ι denote the inclusion f −1 (t)∩Bε
→ Bε . The composition r ◦ ι gives a map f −1 (t)∩Bε −→ V ∩Bε , which induces a morphism in homology (called a “specialization map”). This morphism carries Hn (F ) into Hn (V ∩ Bε ) ∼ = 0, so the µ generators of Hn (F ) vanish. Thus we call them vanishing cycles. 7.5 Remarks on Milnor numbers and vanishing cycles. For many purposes it is important to have preferred sets of vanishing cycles, as for instance to be able to control the bilinear pairing in Hn (F ) given by the intersection product, and to compute its signature (c.f. §8 below and IV.2). For the Pham-Brieskorn polynomials, in some cases the join of Pham can be used to construct a preferred set of vanishing cycles. In [131] Lˆe constructs an equivalent polyhedron for all singularities as above (and in the more general setting of his fibration theorem that we explain below). However the problem of finding a “distinguished basis” of vanishing cycles is indeed an issue and this can be used to obtain deep information about the singularities. We refer to [41, 43, 68, 85, 126] for very interesting results on this subject.

I.7. The join of Pham and the Milnor number

29

It is worth noting that the remarkable feature about Hamm’s fibration theorem (for holomorphic map-germs which define isolated complete intersection singularities, see [95, 146]), is that the topology of the fibres behaves as in the case of hypersurfaces, in the sense that the fibres have the homotopy type of a bouquet of spheres of middle dimension. Thus one has a well-defined “Milnor number” for isolated complete intersection singularities. For curve singularities a notion of the Milnor number was introduced by R.-O. Buchweitz and G.-M. Greuel in [47]. For a smoothable curve singularity (C, 0) (i.e., when there is a manifold “playing the  for a smoothing C  of the singularity, role of a Milnor fibre”) it is equal to 1 − χ(C) χ(·) is the Euler-Poincar´e characteristic. However, there exist surface singularities which have smoothings with different Euler-Poincar´e characteristics ([204]). This does not permit us to generalize the notion of the Milnor number to higher dimensions so that for smoothable singularities it has the usual expression in terms of the Euler characteristic. In [72] is introduced a notion of Milnor number for every isolated singularity in an analytic variety. This is defined as the difference of two indices associated to every holomorphic 1-form on an isolated singularity germ, but it does not depend on the choice of the 1-form. One of these indices corresponds to the “1” in the difference (1 − χ(F )) in (iv) above, the other corresponds to χ(F ). The first index is the so-called “radial index” defined in [69, 70, 71, 35, 3, 233]. The second index is inspired in the homological index of Gomez-Mont in [90] and the index for 1-forms defined in [69, 70, 71] in analogy with the so-called GSV-index of [88, 232, 3]. In the case when the ambient variety V is a complete intersection germ in Cn+k , this homological index coincides with the number of zeroes (counted with multiplicities) of an extension of the 1-form to a Milnor fibre of V . There are also several possible generalizations of the concept of “Milnor number” for map-germs defined on singular spaces. The first one comes from Lˆe’s fibration theorem in [129], by defining µ(f ) to be (−1)n+1 (1 − χ(F )), where n is the dimension of the ambient space and F is the Lˆe-Milnor fibre of f . A second definition comes from [90, 173] for functions on curve singularities, and from [108] for functions on isolated complete intersection germs in general, of any dimension. There is a third generalization coming from [36] by means of the Nash blow-up. These different generalizations of the Milnor number are compared in [234]. The concepts of Milnor number and vanishing cycles have also been extended to varieties with non-isolated singularities. On the one hand the vanishing cycles have given rise to the so-called “vanishing homology” when the singular set has dimension more than 0. We refer to [236, 237, 252, 253, 154, 155, 156, 158, 243, 221] for outstanding results in this direction and for very general “bouquet theorems” for the Milnor fibre. On the other hand, Parusinski defined in [195] a “generalized Milnor number” associated to each connected component of the singular set of a hypersurface in a complex manifold. Parusinski’s Milnor number was extended in [35] to local complete intersections, and this generalization is inspired in the interpretation (iv) of the Milnor number in Theorem 7.4, together with the above comments on indices of vector fields on singular varieties. In fact, using that language one can prove ([232]) that the Milnor number of an isolated

30

Chapter I. The Classical Theory

complete intersection singularity is obtained, up to sign, by localizing at the singularity two different characteristic classes associated to singular varieties: one is the top-dimensional Schwartz-MacPherson class, the other is the top-dimensional Fulton-Johnson class. When the singular set S has dimension s > 0, one can also localize other Schwartz-MacPherson and Fulton-Johnson classes (see [34, 35] for more about these classes). The differences amongst them define the so-called Milnor classes µi (Sj ) ∈ H2i (Sj ; Z), where the Sj are the connected components of the singular set S and i = 0, . . . , sj = dim Sj . We refer to [5, 34, 37, 35, 196, 221, 266] for different viewpoints of the Milnor classes and characteristic classes of singular varieties; [221] relates these with the vanishing cycles.

I.8

Exotic spheres and the topology of the link

We recall that a complex analytic variety V is (analytically) normal at a point x if every bounded holomorphic function on a punctured neighborhood of x extends to a holomorphic function at x (see for instance [179]). Normality at a point implies that the singular set is locally of codimension at least 2 (for a hypersurface or complete intersection germ this condition is actually equivalent to normality). D. Mumford proved in [176] that a 2-dimensional normal complex analytic variety is a topological manifold iff it is a complex manifold. More precisely, Mumford proved that if x is a normal point in a complex surface V and the link K of x is simply connected, then K is the 3-sphere S3 and V is a smooth complex manifold at x. This remarkable theorem motivated a lot of research on the topic by various authors, most notably by E. Brieskorn [40, 39], F. Hirzebruch [102, 103] and J. Milnor [168, 167, 166], and it was shown to be completely false in higher dimensions. In fact Brieskorn showed in [40] that for every k > 1 odd, the link of 0 in the algebraic hypersurface 2 + zn3 = 0 z02 + z12 + · · · + zn−1

which is a generalized trefoil knot, is homeomorphic to the usual sphere S2n+1 . It was thus natural to ask whether this link is diffeomorphic to S2n+1 or if it is an “exotic” sphere. Let us recall that a smooth n-dimensional closed oriented manifold σ n is a topological sphere if it is homeomorphic to the standard unit sphere Sn . Smale’s theorem [238] implies that for n ≥ 5 the manifold σ n is a topological sphere iff it is a homotopy sphere, i.e., if it has the same homotopy groups as the n-sphere; σ n is said to be an exotic sphere if it is homeomorphic but not diffeomorphic to Sn . The first exotic sphere was found in dimension 7 by Milnor [165] in 1956, and it was later shown by Kervaire and Milnor in [113] that there are no exotic spheres in dimensions less than 7 and = 3, 4. In dimensions 3,4 the existence of exotic spheres is the famous Poincar´e’s conjecture and the methods of [113] break down. Two differentiable structures on spheres of dimension ≥ 5 are said to be equivalent if there is an orientation preserving diffeomorphism among them. It is

I.8. Exotic spheres and the topology of the link

31

proved in [113] that for n ≥ 5 these equivalence classes form a finite Abelian group Θn , with the connected sum as operation; the identity element is the standard Sn . This group contains a “preferred subgroup”, usually denoted bPn+1 ⊂ Θn , of those homotopy spheres that bound a parallelizable manifold. This group is zero for n even and for n = 3 odd it is a finite cyclic group which has finite index in Θn . This cyclic group has order 1 or 2 for n ≡ 1 (mod 4), but for n ≡ 3 (mod 4) its order |bP4m | grows more than exponentially:



4Bm  ) , |bP4m | = 22m−2 (22m−1 − 1) · numerator of ( m where the Bm are the Bernoulli numbers. Thus for instance (see [113]), for n = 7, 11, 15 or 19 there are, respectively, |bPn+1 | = 28, 992, 8128 and 130816 nonequivalent differentiable structures on the n-sphere, that bound a parallelizable manifold. We observe that the link K 2n−1 of an isolated complex hypersurface singularity has a canonical differentiable structure, being the intersection K = (V − 0) ∩ Sε of two smooth submanifolds of Cn+1 , and with this differentiable structure it is the boundary of the Milnor fibre, which is a parallelizable manifold. Hence if one such link K is a topological sphere, then it necessarily represents an element in bP2n . Notice that the link K 2n−1 of an isolated complex hypersurface singularity (V, 0) in Cn+1 is always (n − 2)-connected, by [168]. Thus, if n > 2 then its homology vanishes in dimensions i = 1, . . . , n−2. Hence, by the Poincar´e duality isomorphism, its homology also vanishes in dimensions n + i, i = 1, . . . , n − 2; so the only possibly non-zero groups are in dimensions i = n, n − 1 and of course i = 0, 2n − 1 where it is the integers (or the corresponding ring of coefficients). If Hn−1 (K) vanishes then Hn (K) also vanishes and K is a homology sphere. If n ≥ 2, then K is simply connected by [168] and therefore it is a homotopy sphere whenever it is a homology sphere. Furthermore, if n ≥ 3 then Smale’s theorem [238] on Poincar´e’s Conjecture in dimensions ≥ 5 implies that K is actually homeomorphic to S2n−1 . Summarizing, for n = 1, 2 there are no exotic spheres appearing as the link of an isolated complex singularity. For hypersurfaces of dimension n ≥ 3 the link is a topological sphere iff Hn−1 (K) vanishes (c.f. Lemma 8.2 in [168]). In order to have a criterion to decide when this group vanishes, recall one has the monodromy h of the corresponding Milnor fibration, also called the characteristic homeomorphism of the fibre. This is the first return map on the fibres, defined by lifting to S2n+1 −K a vector field on the base S1 . Fixing a fibre F0 , this map induces a homomorphism h∗ : Hn (F0 ) → Hn (F0 ) , known as the monodromy representation of the fundamental group π1 (S1 ) of the base. Using this one can define the characteristic polynomial of the fibration: ∆(t) := det(tId∗ − h∗ ) , which is a polynomial of the form: tµ + b1 tµ−1 + · · · + bµ−1 t ± 1 ,

32

Chapter I. The Classical Theory

with integer coefficients, since Hn (F ) is free Abelian of rank µ. This is an ndimensional generalization of the Alexander polynomial of a knot. The following theorem was proved in [40, Satz 1.(i)] for the Pham-Brieskorn polynomials and by Milnor [168, Theorem 8.5] for isolated hypersurface singularities in general: 8.1 Theorem. For n ≥ 3, K is a topological sphere if and only if the integer ∆(1) = det(I∗ − h∗ )

is

± 1.

To prove this result one uses the Wang sequence associated to the Milnor fibration (see p. 67 in [168]): h −I

∗ ∗ Hn (F0 ) −→ Hn (F0 ) −→ Hn (Sε − K) −→ 0 .

This exact sequence implies that ∆(1) = ±1 if and only if Hn (Sε − K) vanishes. The result then follows from the isomorphisms below, which are given by the Alexander and Poincar´e duality isomorphisms, respectively: Hn (Sε − K) ∼ = H n (K) ∼ = Hn−1 (K)




For instance consider the link K(a0 ,...,an ) of the Brieskorn singularity: z0a0 + z1a1 + · · · + znan = 0 . It follows from the previous discussion about Pham’s work (also from [39, 168]) that the nth homology group Hn (F ) of the Milnor fibre is free Abelian of rank: µ = (a0 − 1)(a1 − 1) · · · (an − 1) . It is shown in [39] that the roots of the characteristic polynomial h∗ : Hn (F ; C) → Hn (F ; C) are the products (ω0 ω1 · · · ω n+1 ), where each ωj runs over the ath j roots of unity other than 1. Hence the characteristic polynomial is given by: ∆(t) = (t − ω0 ω1 · · · ωn ) . As an example consider the case: 2 + zn3 = 0 , z02 + z12 + · · · + zn−1

so that a0 = · · · = an−1 = 2 and an = 3. The roots of h∗ are: √   −1 ± −3 ω0 = · · · = ωn−1 = −1 , ωn = . 2

I.8. Exotic spheres and the topology of the link

33

Hence: ∆(t) = t2 − t + 1 2

∆(t) = t + t + 1

for n odd ; for n even .

Thus K(2,··· ,2,3) is a topological sphere when n is odd, i.e., when the link has dimension 2n − 1 = 1, 5, 9, . . . , as claimed earlier. In dimensions 1 and 5 there are no exotic spheres, so K has to be the usual sphere. However in dimension 9 there is one exotic sphere and this is represented by the generalized trefoil knot K, but in order to decide this one has to use rather sophisticated arguments. Indeed it was shown in [113] that the elements in bP2n−1 that bound a parallelizable manifold F which is (n − 1)-connected are distinguished by (see IV.1 below for the definition of this invariant): (i) the signature σ(F ) ∈ Z of the intersection pairing in Hn (F ), if n = 2 is even; or (ii) the Arf-Kervaire invariant c(F ) ∈ Z2 , if n is odd. In the case n = 2m one has that the intersection pairing in Hn (F ) is even and therefore its signature σ(F ) must be divisible by 8; if gm represents the generator of bP4m then (by [113]) one has that K represents the element

σ(F )  · gm . 8 When n is odd a remarkable theorem of J. Levine [137] says the Kervaire invariant c(F ) is 0 if ∆(−1) ≡ ± 1 mod 8, and it is 1 if ∆(−1) ≡ ± 3 mod 8. So the differentiable structure on K is determined by the characteristic polynomial ∆(t). As an example of these results Brieskorn shows in [39, p. 11] that the link of the n-dimensional singularity V(2,...,2,d) , with n, d ≥ 3 odd numbers, is a topological sphere and the structure on the link is exotic iff d ≡ ±3 mod 8. Also using these results Hirzebruch was able to prove in [103, p. 20-21] that the link of the singularity K≈

2 = 0, z03 + z16k−1 + z22 + · · · + z2m

k ≥ 1; m ≥ 2

is a topological sphere which represents the element: (−1)m k gm ∈ bP4m . As an example Hirzebruch remarks that for m = 2 one has that bP8 is the whole group Θ7 and one gets the 28 classes of differentiable structures on S7 by taking k = 1, . . . , 28. Similarly bP12 = Θ11 and k = 1, . . . , 992 gives all the differentiable structures in S11 . More generally, Brieskorn proved in [39, Korollar 2]: 8.2 Theorem (Brieskorn). Every homotopy sphere of dimension m = 2n − 1 > 6 that bounds a parallelizable manifold is the link K(a0 ,...,an ) for some hypersurface singularity of the form z0a0 + z1a1 + · · · + znan = 0 .

Chapter II

Motions in Plane Geometry and the 3-dimensional Brieskorn Manifolds This chapter is about the theorems of Klein (1884) and Milnor (1975), relating certain groups of isometries in plane geometry to the Brieskorn varieties V(p,q,r) = {z1p + z2q + z3r = 0}. There are three cases, according as the triple p−1 + q −1 + r−1 is greater than, equal to or smaller than 1. The theorems say that in either case the link M(p,q,r) is diffeomorphic to a space of orbits of the form Γ\G, where G is a 3-dimensional Lie group and Γ is a discrete subgroup of G. In the first case, when p−1 +q −1 +r−1 > 1, the group G is SU (2), the universal cover of SO(3), the group of rotations of the 2-sphere; SU (2) is isomorphic to the group of unit quaternions, so it is diffeomorphic to the 3-sphere, and its discrete subgroups are either cyclic or the lifting to SU (2) of a dihedral group or a group of symmetries of a platonic
solid. When p−1 + q −1 + r−1 < 1 the group G is SL(2, R), the universal cover of P SL(2, R), the group of orientation preserving isometries of the hyperbolic plane H2 , and the subgroups in question come from the celebrated Schwarz triangle groups of isometries in hyperbolic plane geometry. The case p−1 + q −1 + r−1 = 1 is somehow different and we discuss that difference in Chapter III. We start this chapter with an exposition of the groups of symmetries of the platonic solids from the classical point of view and we relate them (in §2) to the Schwarz triangle groups; for this we explain briefly the groups of isometries in the three classical plane geometries: Euclidean, spherical and hyperbolic. Then we discuss the Lie group structure on the sphere S3 , both as unit quaternions or as the group SU (2) of unitary transformations, and relate this to the group SO(3) of rotations of the 2-sphere. In Section 4 we state Klein’s theorem for the finite subgroups of SU (2) and we give Milnor’s proof of a refinement of Klein’s theorem, which expresses beautifully the relation between the finite subgroups of SO(3) and

36

Chapter II. On the 3-dimensional Brieskorn Manifolds

the Brieskorn singularities in the spherical case. We then speak about the group
P SL(2, R) and its universal cover SL(2, R), giving Milnor’s interpretation of the latter as “labeled biholomorphic maps” of the upper half-plane. This is then used to explain Milnor’s theorem for the Brieskorn singularities in the hyperbolic case.

II.1

Groups of motions in the 2-sphere. The polyhedral groups

In this section we study and describe the finite subgroups of SO(3), the group of orientation preserving isometries of the 2-sphere, endowed with the metric induced from R3 . This is a well-understood subject that has fascinated generations of mathematicians since long ago. In fact the classification of the simplest of such groups, those coming from Euclidean plane geometry, is usually attributed to Leonardo da Vinci. It seems that J.F.C. Hessel (1830) was the first to enumerate the finite 3-dimensional groups of symmetries, which were also studied later by A. Cayley, F. Klein, H. Poincar´e and many others. More recently, the contributions to the subject by H.S.M. Coxeter in the 20th century are remarkable. The literature in this topic is vast and we could just refer to it. However we prefer to say a few things here, for the sake of completeness, to motivate similar constructions for the group P SL(2, R), and for the joy of discussing a little about this important and very beautiful piece of mathematics. This also helps to set up the notation which can be confusing sometimes, since the groups in question can be considered in SO(3), or lifted to the trivial double cover O(3), or extended to the universal cover SU (2). Those wanting to go deeper into the subject can look at [56, 55], or also at [171] where Milnor presents these groups from his own viewpoint, which is very elegant and closer to what we need for this text (see also [118]). Consider first the orthogonal group O(3) of linear transformations of R3 that preserve the usual metric in R3 , given by the standard quadratic form (1, 1, 1). This is the group generated by reflections on all 2-planes through the origin and it can be identified with the space of orthonormal 3-frames in R3 ; its elements are matrices with determinant ±1. It has a preferred subgroup SO(3) consisting of those frames which induce the usual orientation in R3 , i.e., matrices with determinant 1 and the corresponding linear transformations of R3 preserve the orientation. This is the subgroup of O(3) consisting of words of even length, because each reflection reverses the orientation. Since the composition of two reflections is a rotation, it follows that SO(3) is the group of rotations of S3 . The group SO(3) acts transitively on the unit 2-sphere S2 ⊂ R3 . The stabilizer (or isotropy) subgroup of each point is S1 ∼ = SO(2) ⊂ SO(3). This means that given any pair of points x1 , x2 ∈ S2 , and unit tangent vectors v1 ∈ Tx1 S2 , v2 ∈ Tx2 S2 , there is exactly one element in SO(3) taking x1 into x2 and v1 into v2 . Therefore SO(3) can be identified with the unit tangent bundle of S2 . Hence, topologically, SO(3) is an S1 -bundle over S2 with Euler class 2. One has an identification S2 ∼ = SO(3)/SO(2). It is worth saying that every compact Lie group

II.1. Groups of motions in the 2-sphere

37

has an essentially unique bi-invariant metric (i.e., invariant under left and right multiplication by elements in the group), and the usual round metric on S2 comes from the bi-invariant metric on SO(3) via the identification S2 ∼ = SO(3)/SO(2). We are interested in the finite subgroups of SO(3). These are the cyclic groups of finite order, the (finite) dihedral groups and the polyhedral groups of symmetries of a tetrahedron, an octahedron and an icosahedron. Let us briefly introduce these groups (we refer to Klein’s beautiful book [114] for more on the subject). a) The cyclic groups Cr of any finite order r > 0. These are embedded in SO(3) as subgroups of SO(2) ∼ = S1 , the group of rotations of R2 (which is identified with 3 the xy-plane in R ); Cr can be regarded as the group of symmetries of a regular polygon Pr in R2 with r-edges. b) The dihedral groups Dr of order 2r > 1. These are obtained from the cyclic groups by adding a generator of order 2. This generator can be represented by a rotation α in R3 by an angle of 180o around the x-axis, taking the z-axis into itself but with reversed orientation. Restricted to the xy-plane this is a reflection, but it is a rotation in R3 . Its order is twice the order of the corresponding cyclic group. There is a geometric way to “visualize” this group by looking at its action on the 2-sphere; this is convenient for later discussions. Start with a regular polygon Pr (including its interior) in the xy-plane with r-edges, and let Pr denote its boundary. Let us join the r vertices of Pr with the centre 0 by r straight lines, getting a triangulation of Pr with r triangles. Now project Pr radially from the centre to get the unit disc D2 in the xy-plane, whose boundary S1 corresponds to Pr . The disc D2 inherits a triangulation from that of Pr . Next, make a stereographic projection from the south pole S = (0, 0, −1). This carries D2 into the upper half of the sphere S2 , which is covered by r congruent and symmetric triangles that we denote {T1 , . . . , Tr }. The equator S1 = ∂D2 is pointwise fixed by this projection and the origin 0 is mapped to the north pole (0, 0, 1). It is clear that each triangle is bounded by geodesic arcs in the sphere (for the usual round metric), with two right angles at the vertices where the edges meet the equator S1 . All the r triangles meet at the north pole, so each has an angle 2π/r there. Thus the angles of each Ti 2 are ( π2 , π2 , 2π r ). We notice that the action of Cr on S permutes these triangles. The generator α carries them into the southern hemisphere of the sphere, which gets equipped with a triangulation σ. All together we have 2r triangles in σ covering the sphere and bounded by geodesics. It is an exercise to show that either of these triangles works as the fundamental domain for the action of Dr . That is, if we pick any one of them and denote it T, then the 2r images of T by the elements of Dr cover the whole sphere and provide the above triangulation. We remark that later in this section we relate these groups with the “triangle groups” (p, q, r), and for this it will be important to look at the angles ( π2 , π2 , 2π r ) of the triangles in σ. To construct the triangle groups it is however convenient to consider the full group of reflections on the edges of a given triangle, thus getting

38

Chapter II. On the 3-dimensional Brieskorn Manifolds

subgroups of O(3). The order of these groups is twice the order of the corresponding groups of rotations. Thus, in order to get a triangle which is a fundamental domain for the action of the full triangle group, we must double the number of triangles in the triangulation σ above. This is achieved by splitting each triangle Ti in two equal halves by joining the north and south poles by r geodesic arcs in S2 passing through the middle points of the r edges of Pr , which are contained in the equator. Hence the triangles will have angles ( π2 , π2 , πr ), corresponding to the triangle group (2, 2, r) as we indicate below. c) The tetrahedral group. This is the group of rotations in R3 that preserve a regular tetrahedron T , which is a pyramid with triangular base. It is a group of order 12. Notice that each symmetry of T must carry vertices into vertices, middle points of the faces into middle points of the faces and the same for the edges. We may construct the 12 elements of this group as follows. Let (v1 , . . . , v4 ) be the vertices of T and let (f1 , . . . , f4 ) be the faces, numbered in such a way that fi is the face opposite to the vertex vi . For each i, let mi ∈ fi be its barycentre (the middle point), and let li be the line determined by vi and mi . Then rotating by an angle 2π/3 along li as axis we get a symmetry of T , that we denote ρi ; it has order 3. Doing this for each of the four vertices we get the 12 elements of T (a combination of all these rotations produces the identity in the group). To get a fundamental domain for the corresponding action on the sphere is rather easy: take a triangulation of the faces by joining the point mi by lines with each of the three vertices in the face fi . We get a triangulation of T with 12 triangles. Now make a radial projection from the centre 0, mapping T into S2 . The vertices (v1 , . . . , v4 ) are fixed points of this projection. We get a triangulation σ of the sphere with 12 triangles. Anyone of these serves as fundamental domain. We may compute the angles of these triangles as follows. Each triangle T in σ has two vertices amongst (v1 , . . . , v4 ) and another vertex at the barycentre of one of the faces (f1 , . . . , f4 ). At each vertex of σ corresponding to a vertex vi of T we have six triangles of σ, two for each face of T containing the given vertex. Hence the angle of each triangle at this point is π/3. At a vertex of σ corresponding to the barycentre of a face fj we have three vertices of σ, so the corresponding angles are 2π/3. Thus the angles of each Ti in σ are ( π3 , π3 , 2π 3 ). Observe that we have considered only symmetries of the tetrahedron given by rotations. There are also symmetries given by reflections. For instance, given a face fi of T , take the middle point mij of an edge eij of fi , the vertex vj in fi opposite to the edge eij , and the vertex vi of T opposite to the face fi . The points vi , vj and mij determine a plane P which cuts the tetrahedron in two equal halves. The reflection in P determines a symmetry of T . Starting with each vertex of T we get three symmetries in this way, corresponding to the edges in the opposite face of T . This gives 12 more symmetries of T , which together with the previous 12 rotations gives the full group of symmetries of the tetrahedron, with 24 elements. In this case the fundamental domain for the action on the sphere is obtained by dividing in two each triangle of the above triangulation σ in an appropriate way. For this, take the full barycentric triangulation of the faces of

II.1. Groups of motions in the 2-sphere

39

T . That is, for each face fi take its barycentre mi and join it by lines with the three vertices of fi as before (this gives the triangulation considered above), and also join the barycentre mi with the middle points mij of the three edges of fi . This corresponds to intersecting T with its planes of symmetry. Now we have a triangulation of T with 24 symmetric triangles Ti . Map these to the sphere by radial projection and get a triangulation of S2 with 24 congruent and symmetric triangles. Let us measure the angles of these triangles. At the vertices which are also vertices of T the angle is still π/3; each triangle has now one such vertex. At the vertices which are middle points of the faces we now have six adjacent triangles, since each of the previous three triangles was divided in two. Thus the angle is also π/3. Finally, at the vertices corresponding to the middle points of the edges we have four adjacent triangles, so the angle is π/2. Hence the triangle T ⊂ S2 which serves as fundamental domain for the action of the full group Σ∗ of reflections of the tetrahedral group has angles ( π2 , π3 , π3 ). This corresponds to the triangle group (2, 3, 3). It remains to prove that the triangle T is indeed a fundamental domain for the action of Σ∗ . This is more easily seen by looking directly at the action on the tetrahedron: it is clear that the rotation ρi above permutes the three triangles T (which are double triangles) in the face fi , and it also permutes the three faces adjacent to the vertex vi . Thus, iterating this rotation ρi and combining it with a rotation ρj around some other vertex, we see that the images of the face fi cover all of T and the image of a single triangle T is carried to the 12 triangles of this triangulation. This proves that T is a fundamental domain for the action of Σ ⊂ Σ∗ , the group of rotations of T . To complete the proof we notice that by construction, T is divided in two symmetric triangles Ti by a plane of symmetry of T , and the reflection in that plane permutes these two triangles. Hence T is a fundamental domain for the action of Σ∗ . d) The octahedral group. An octahedron O is the polyhedron whose vertices are the middle points of the faces of a cube C. It has six vertices (one for each face of C), 12 edges and eight (triangular) faces. The middle points of these eight faces of O are vertices of a smaller cube. Hence the cube and the octahedron are dual polyhedra and their groups of symmetries are equal. The full group of symmetries of the cube has 48 elements. This is a subgroup of O(3). The octahedron O has nine planes of symmetry, i.e., planes in R3 with respect to which the reflection maps O into itself. Six of these planes are the following (see Figure 6). Each vertex, say v1 has four adjacent vertices, say v2 , . . . , v5 and an opposite one v6 , and the four adjacent vertices determine a quadrangle with edges e12 , e13 , e14 , e15 . Suppose e12 and e14 are opposite edges in the quadrangle, and similarly for e13 and e15 . Let m12 denote the middle point of the edge e12 . Then the points v1 , v6 , m12 determine a plane of symmetry of O (which passes by m14 ), and the points v1 , v6 and m13 determine another plane of symmetry. There are three pairs of opposite vertices and each such pair determines two planes as above, so we have six planes. These divide each triangular face of O into six al-

40

Chapter II. On the 3-dimensional Brieskorn Manifolds

Figure 6: An octahedron and one of its planes of symmetry.

ternately congruent and symmetric triangles that have the middle point of the face as common vertex; they meet at each vertex of O in sets of eight and in sets of four at each middle point of the edges. Hence, if we put O with its vertices in the unit 2-sphere S2 and map O into S2 by radial projection from the origin, we get a triangulation of the sphere by triangles whose angles are π/2 (for the vertices which come from middle points of the edges), π/3 (for the vertices that correspond to middle points of the faces), and π/4 (for the vertices which are also vertices of O). So this corresponds to the triangle group (2, 3, 4). Any one of these 48 triangles T serves as fundamental domain for the action of the full octahedral group Σ∗ in S2 . To get a fundamental domain for the corresponding group Σ of rotations, which has 24 elements, we must take a “double” triangle T as before. We leave it as an exercise to show that these triangles T and T are indeed fundamental domains for the actions of Σ∗ and Σ, respectively. e) The icosahedral (and dodecahedral) group. Just as the octahedron and the cube are dual polyhedra with the same groups of symmetries, so too the icosahedron and the dodecahedron are also dual: the middle points of the 12 pentagonal faces of the dodecahedron determine the 12 vertices of the icosahedron, and the middle points of the 20 triangular faces of the latter are the vertices of the former. Each has 30 edges. Here is a method for actually making a dodecahedron (see Figure 7 below): start with a pentagon P0 and put it flat over a table. Now attach five pentagons P1 , . . . , P5 to it by gluing each of them to one of the sides of P0 . And now glue each pentagon P1 , . . . , P5 to its two neighbors by the edge next to each of them.

II.1. Groups of motions in the 2-sphere

41

In order to actually do this physically, you will have to fold them by the edge in common with P0 , as if these edges were the hinges of a door. The result looks like a bowl with boundary a regular polygon with 10 edges (the two free edges of each pentagon P1 , . . . , P5 ). Now make a second bowl like this and glue them together along their boundary. The result is a dodecahedron D.

Figure 7: Making a dodecahedron.

Let us look at some of its planes of symmetries: we need 30 such planes to get the triangulation that gives the fundamental domain for the full group of symmetries of D. Notice that each face has exactly one opposite face, so everything we do in one side gets automatically done in the other side. That is, we only look at one of our two bowls used to construct D; this has six faces in it. Let us set D in R3 so that its vertices are in S2 . Fix a face f1 of the dodecahedron. Given one of its vertices v, take the middle point m of the opposite edge in that face. The points v and m determine a unique plane in R3 passing through the origin, and the reflection in this plane carries D into itself. Notice this plane cuts the face f1 in two equal halves. The 3-sphere Doing this for the five vertices in f1 we get a triangulation of this face with 10 congruent, symmetric triangles with a common vertex at the middle point of the face. Doing this for all the faces we get a triangulation of D with 120 triangles. Now map these to the sphere by radial projection. At each vertex in the sphere which is a vertex of D, one has six triangles with that common vertex, hence they have angle π/3 there. As before, at each vertex coming from a middle point in an edge of D we have four triangles, so the angle is π/2. Finally, at each vertex coming from a middle point of the faces we have the 10 triangles, so the angle is π/5. Thus each of these 120 triangles has angles ( π2 , π3 , π5 ). This corresponds to the triangle group (2, 3, 5). Let us show that anyone of these triangles serves as fundamental domain for the action of the full group of reflections on S2 . As in the case of the tetrahedron, by construction it is enough to prove that the double triangle T is a fundamental domain for the corresponding group of rotations. For this we observe that the action is transitive on the triangles T contained in the same face (just take the appropriate rotation with axis the line determined by the middle point of the face and that of the

42

Chapter II. On the 3-dimensional Brieskorn Manifolds

opposite face). Furthermore, this rotation permutes the five pentagons adjacent to the given face, as well as with the other five pentagons adjacent to the opposite face. Hence the proof is complete by noticing that given any two opposite faces of D, there is always a rotation permuting these faces.

II.2

Triangle groups and the classical plane geometries

In this section we describe the triangle groups, which were introduced by H.A. Schwarz and W. Dyck in the 1880s and are related to the polyhedral groups of Section 1. For this we consider the three classical 2-dimensional geometries: Euclidean, spherical and hyperbolic. Let us recall briefly a few facts about these geometries. We recall that the group of Euclidean motions (isometries) of R2 is the affine group E(2), isomorphic to O(2)×R2 , where O(2) is generated by the reflections on lines through the origin in R2 , and R2 is identified with its group of translations. Since translations are given by reflections on parallel lines, the group E(2) is generated by the reflections on all possible lines in R2 . In particular, given a triangle T ⊂ R2 with angles (α, β, γ), one has the group of isometries of R2 generated by the reflections on the edges of T . If two edges e1 , e2 of T enclose the angle α at the vertex v1 , then the reflections on these two lines yield a rotation around v1 by an angle of 2α, and similarly for the other angles. Hence if the angle is of the form π/p for some p ∈ Z, p > 1, then repeating p-times the reflections on these two lines brings T back to its original position, while the 2p-images of T fill out a neighborhood of the given vertex. We leave it as an exercise to think what happens when the angles are not of this form. (There are two remarkably different situations: when the angles are “rational”, so that after a number of times the triangle comes back to its original place, and when the angles are “irrational”.) Now let us see what happens when we are on the 2-sphere. We know that its group of isometries is O(3), which is generated by the reflections on all 2-planes through the origin in R3 . Notice that each such plane meets S2 orthogonally in a circle of maximal length, i.e., an equator. These circles are the geodesics in S2 , i.e., the curves that locally minimize the distance between points. Conversely, given any geodesic arc on the sphere, we know that this corresponds to a circle of maximal length; this circle is necessarily the intersection of S2 with a 2-plane through the origin in R3 and we can define a reflection on this plane. Hence, as in the Euclidean case, given a triangle T in S2 , bounded by geodesics, we can look at the group of isometries of S2 generated by reflections on the edges of T . As before, if a given angle of T is of the form π/p for some p ∈ Z, p > 1, then the reflections on the corresponding edges produce a rotation of an angle 2π/p, and repeating this p-times brings T back to its original position, while its 2p-images fill out a neighborhood of the rotation point. Now we move to hyperbolic geometry. As models for H2 , the 2-dimensional hyperbolic plane, one can take the Poincar´e model of the unit disc, the Lobachev-

II.2. The triangle groups

43

sky model of the upper half-plane, or whatever model one prefers. For this section I take the Poincar´e model, so that H2 is the unit disc in R2 with the hyperbolic metric. Here is a way to define this metric. This elegant construction, which I learned from D.P. Sullivan, works in all dimensions and has the additional advantage that it gives for free the corresponding group of isometries. Let M¨ ob(3, R) denote the group of transformations of the extended plane  2 = R2 ∪ {∞} ∼ R = S2 , generated by inversions in all possible circles, including the reflections in straight lines, which are circles of infinite radius. Denote by M¨ob(2, R) the subgroup of M¨ ob(3, R) generated by inversions that preserve the unit disc D = D2 ⊂ R2 . The  2 that meet generators of M¨ ob(2, R) are the inversions in circles (and lines) in R 1 orthogonally the boundary circle S = ∂D. It is an exercise to see that M¨ob(2, R) ◦

acts transitively on the interior D of D. Furthermore, the isotropy subgroup at ◦

0 ∈ D is generated by the reflections on lines through the origin, so it is the group O(2). Hence we can put the usual Riemannian metric on the tangent plane ◦

◦

T0 (D), tangent to D at 0, and spread this metric around using the group action. ◦

That is, given a point x ∈D, take an element g in M¨ ob(2, R) with g(0) = x; the ◦ ◦ ∼ T0 (D) and the metric on the derivative of g identifies the tangent planes Tx (D) = latter induces a metric in the former. This is well defined because the ambiguity lies in O(2) and this group preserves the standard metric. Thus one has a well◦

defined Riemannian metric on D and it is an exercise to show that this metric is of constant negative curvature, so it gives the Poincar´e model for hyperbolic ◦

geometry. By construction we see that the geodesics are circles in D that meet the boundary S1 orthogonally, and the isometries are compositions of inversions ◦

(that we shall call also “reflections”) on these circles. By H2 we mean D with this metric. Notice that M¨ ob(2, R) has the subgroup M¨ ob+ (2, R), of orientation preserving isometries of H2 , as an index 2 subgroup, consisting of the words of even length. obius This group, also denoted Iso+ (H2 ), is isomorphic to the famous group of M¨ transformations z → az+b with real coefficients and ad − bc = 1. It is also isocz+d morphic to the group P SL(2, R) := SL(2, R)/ ± 1 = GL(2, R)/R∗ . The previous discussion shows that as a manifold, M¨ob(2, R) is diffeomorphic to H2 × SO(2), ◦

so it is an open solid torus S1 × D. As before, given a triangle T in H2 bounded by geodesics, we can look at the group of isometries generated by the reflections (inversions) on the edges of T , and given an angle of T of the form π/p for some p ∈ Z, p > 1, the reflections on the corresponding edges produce a rotation of an angle 2π/p, and repeating this p-times brings T back to its original place.

44

Chapter II. On the 3-dimensional Brieskorn Manifolds

From now on we essentially follow the exposition given in [171, §2]. Let us denote by P either the Euclidean plane (i.e., R2 with the Euclidean metric), the 2-sphere with its usual metric, or the hyperbolic plane. Given integers p, q, r ≥ 2, there is a triangle T = T(p,q,r) in P with angles π/p, π/q, π/r. This triangle is Euclidean if p1 + 1q + 1r = 1, spherical if this sum is > 1 and hyperbolic if the sum is < 1. In the first case the area of T can be arbitrary, but in the spherical and hyperbolic cases the area is determined by the angles: it is | πp + πq + πr − π | . 2.1 Definition. (i) The full (Schwarz) triangle group Σ∗ = Σ∗(p,q,r) is the group of isometries of P generated by reflections σ1 , σ2 , σ3 in the three edges of T(p,q,r) . (ii) The (Schwarz) triangle group Σ = Σ(p,q,r) is the index 2 subgroup of Σ∗(p,q,r) consisting of orientation preserving maps. It is customary to denote the triangle groups simply by (p, q, r), but this notation can be confusing in our case, so we use the notation of Definition 2.1. From the previous discussion we know that in the spherical case the group Σ∗ is a subgroup of O(3) and Σ ⊂ SO(3). In the hyperbolic case Σ∗ is a subgroup of M¨ ob(2, R) and Σ is contained in P SL(2, R). In the Euclidean case Σ∗ lives in the affine group E(2) and Σ is contained in E + (2), which is a semi-direct product SO(2)
R2 . The following theorem is due to Poincar´e. 2.2 Theorem. The group Σ∗(p,q,r) has a presentation with generators σ1 , σ2 , σ3 and relations σ12 = σ22 = σ32 = 1

and

(σ1 σ2 )p = (σ2 σ3 )q = (σ3 σ1 )r = 1 ,

where it is understood that the edges e1 and e2 enclose the angle π/p, e2 and e3 enclose the angle π/q, and e3 , e1 enclose the angle π/r. Furthermore, the triangle T(p,q,r) is a fundamental domain for the action of Σ∗(p,q,r) on P . This theorem implies that the various images of the triangle T(p,q,r) cover the whole “plane” and they are pairwise disjoint, except for boundary points. These kinds of decompositions of the “plane” are usually called tesselations, as in Figure 8. That Σ∗(p,q,r) is generated by σ1 , σ2 , σ3 is by definition, and the above discussion shows that these generators satisfy the relations in Theorem 2.2, since each σi is a reflection (so that σi = 1) and the composition of any two of them produces a rotation (around the corresponding vertex) by twice the given angle. So what has to be proved in Theorem 2.2 is that there are not more relations among the generators and that the triangle T(p,q,r) is a fundamental domain, i.e., that its images under the action of Σ∗ on P cover the whole “plane” and they are pairwise disjoint except for the boundaries of the triangles. We refer to [171, §2] for this.

II.2. The triangle groups

45

An immediate consequence of Theorem 2.2 is: 2.3 Corollary. In the hyperbolic and Euclidean cases p1 + 1q + 1r ≤ 1 the group is infinite, while in the spherical case it is finite and its order is given by the area of the sphere divided by that of T :

∗

Σ

(p,q,r)

=

( p−1

+

4 . + r−1 ) − 1

q −1

Recall that the group Σ(p,q,r) of orientation preserving isometries of P consists of the elements of even length in Σ∗(p,q,r) . Let us set (following [171]): τ1 = σ1 σ2 , τ2 = σ2 σ3 , τ3 = σ3 σ1 . One has τ1 τ2 τ3 = σ1 σ2 σ2 σ3 σ3 σ1 = σ1 σ22 σ32 σ1 = 1 and one has another consequence of Corollary 2.3 (see [171, 2.5]): 2.4 Corollary. The group Σ(p,q,r) has a presentation with generators τ1 , τ2 , τ3 and relations τ1p = τ2q = τ3r = τ1 τ2 τ3 = 1 . Let us now list the possibilities. We may assume that 2 ≤ p ≤ q ≤ r. In the Euclidean case p1 + 1q + 1r = 1 there are only three possibilities: the triple (p, q, r) must be (2, 3, 6), (2, 4, 4) or (3, 3, 3). For the spherical case 1p + 1q + 1r > 1 the possibilities are: (2, 2, r), (2, 3, 3), (2, 3, 4) or (2, 3, 5). For all the other infinite triples we are in the hyperbolic case p1 + 1q + 1r < 1. In the spherical case we already know from Section 1 which are the finite subgroups of SO(3); we also know their orders and fundamental domains for their actions on the sphere. This allows us to deduce the following (well-known) theorem. 2.5 Theorem. The finite subgroups of SO(3) are the cyclic groups Cr of finite order and the triangle groups Σ(p,q,r) where (p, q, r) is one of the triples: (i) (2, 2, r), and Σ(2,2,r) is isomorphic to the dihedral group Dr of order 2r. (ii) (2, 3, 3), and Σ(2,3,3) is isomorphic to the tetrahedral group of order 12. (iii) (2, 3, 4), and Σ(2,3,4) is isomorphic to the octahedral group of order 24. (iv) (2, 3, 5), and Σ(2,3,5) is isomorphic to the icosahedral group of order 60. Notice that the dihedral group can also be considered as the group of symmetries of a bi-prism over a regular polygon, so the spherical triangular groups are also called polyhedral groups. In the hyperbolic case 1p + 1q + 1r < 1 the triangle groups are a special case of the so-called Fuchsian groups that we will describe in the next chapter. In the Euclidean case 1p + 1q + 1r = 1 the triangle groups, the analogous group of the quadrangle and Z ⊕ Z, are the only discrete subgroups of E + (2) with compact quotient. We shall come back to this point in Chapter III.

46

Chapter II. On the 3-dimensional Brieskorn Manifolds

Figure 8: Tesselations of S2 and H2 given by the triangle groups Σ∗(2,3,5) and Σ∗(2,3,7) .

II.3. The 3-sphere and its finite subgroups

II.3

47

The 3-sphere as a Lie group and its finite subgroups

This section is about the geometry of quaternions and we refer to [56, Chapter 6] for a clear account of the subject. Let us denote by H the space of quaternions. As a vector space H is R4 endowed with three additional complex structures given by the numbers i = (0, 1, 0, 0),

j = (0, 0, 1, 0) and k = (0, 0, 0, 1),

and defining relations: i2 = j 2 = k 2 = −1 and i j = k ; j k = i ; k i = j . One has i j = −j i ; j k = −k j ; k i = −i k . This defines a multiplication of quaternions in the obvious way, by extending linearly the above operations. Every quaternion can be written uniquely as: q = a0 + a1 i + a2 j + a3 k = (a0 + a1 i) + (a2 + a3 i) j with a0 , a1 , a2 , a3 ∈ R. This gives an identification H ∼ = C2 . In other words, every quaternion q can be written as a pair (z1 , z2 ) of complex numbers, q = z1 + z2 j. Given q = a0 + a1 i + a2 j + a3 k, we say that a0 is its real part, while a1 i + a2 j + a3 k is a pure quaternion. Writing q = a0 + (a1 i + a2 j + a3 k), one defines the quaternionic conjugation q → q¯ by q¯ = a0 − (a1 i + a2 j + a3 k) . The norm q of a quaternion is defined in the usual way, via the identification H ∼ = R4 , and just as for the complex numbers, it is an exercise to verify that quaternionic multiplication satisfies: q2 = q q¯ and

q1 q2  = q1  q2 .

Thus one has that multiplication in H preserves the unit quaternions and induces a Lie group structure on the unit sphere S3 in H. In particular, if we identify H with C2 as above, one has an inclusion C
→ H given by z → (z, 0); quaternionic multiplication restricts to the usual complex multiplication and one has the unit circle S1 as invariant set. This defines an inclusion S1
→ S3 as Lie groups. We notice that we can identify a quaternion q = z1 + z2 j with the complex matrix   z1 z2 A(q) = . −¯ z2 z¯1

48

Chapter II. On the 3-dimensional Brieskorn Manifolds

With this identification one has that the norm of q is the determinant of A(q), thus ψ

one has an induced map: (H − {0}) → GL(2, C) into the 2 × 2 invertible matrices with complex coefficients. Furthermore, the column vectors of A(q) satisfy: (z1 , −¯ z2 ), (z2 , z¯1 ) = z1 z¯2 − z¯2 z1 = 0 , where ·, · is the usual Hermitian product in C. This means that these column vectors form an orthogonal complex 2-frame in C2 ; thus restricting ψ to the unit quaternions, one has that its image consists of all 2 × 2 unitary matrices with determinant 1. That is, we have a canonical identification of S3with the special z1 z2 unitary group SU (2) , consisting of all matrices of the form with −¯ z2 z¯1 determinant 1. With this identification, the circle S1 ⊂ C gets  iθ  embedded in SU (2) e 0 as the group of diagonal matrices of the form . The centre of SU (2) 0 e−iθ is ±I and the projection π S3 ∼ = RP 3 = SU (2) −→ (SU (2)/ ± I) ∼

identifies pairs of opposite points in the sphere. This projection corresponds to the adjoint representation of SU (2) in its Lie algebra, which can be identified with the tangent space at the identity T1 S3 ∼ = R3 . π Let us describe with more care the projection SU (2) → RP 3 . For this we recall that the sphere S2 can be identified with CP 1 , the projective space of complex lines through the origin in C2 . Since SU (2) acts linearly on C2 , its action descends to an action on S2 . This action is by holomorphic transformations, so it preserves the orientation of S2 . Moreover, this action is also transitive, because given any two lines in C2 there is a linear map in SU (2) taking one into the other. The isotropy subgroup consists of the unitary matrices that preserve the z1 -axis, so it is the circle S1 , regarded as the diagonal matrices in SU (2), which is in fact the group U (1). One gets an identification S2 ∼ = SU (2)/U (1) and the fibre bundle projection SU (2) ∼ = S3 → S2 is precisely the Hopf fibration; the fibre over each point x ∈ S2 ∼ = CP 1 is precisely the unit circle in the corresponding complex line. We observe that the metric on S2 is also the one induced by this identification. Thus one gets a homomorphism from SU (2) into the group SO(3) of orientation preserving isometries of S2 , which is actually surjective. Moreover, it is two-to-one, since two matrices in SU (2) induce the same map in CP 1 , the space of lines in C2 , iff they differ by sign. This identifies S3 with the universal cover of SO(3), being a 2-fold cover. Furthermore, since the projection S3 → SO(3) has kernel ±1, it follows that as a manifold SO(3) is the real projective space RP 3 . The theorem below summarizes the previous discussion. 3.1 Theorem. The Lie group S3 of unit quaternions is isomorphic to the special unitary group SU (2). Its centre consists of the quaternions ±1 and the projection π : SU (2) ∼ = S3 → SO(3) is a two-fold cover with kernel the centre ±1 of S3 . One

II.3. The 3-sphere and its finite subgroups

49

has a commutative diagram: ∼ =

Z2 ⏐ ⏐ι 

−−−−→

S1 ⏐ ⏐ π˜

pr1 i −−−1−→ S3 ∼ = SU (2) −−−−→ ⏐ ⏐π 

Z2 ⏐ ⏐ι 

−−−−→ 1 ⏐ ⏐  S2 ⏐ ⏐ Id

pr2 i2 RP 1 ∼ = S1 −−−−→ RP 3 ∼ = SO(3) −−−−→ S2

where the middle line is the Hopf fibration, the bottom line comes from identifying SO(3) with the unit tangent bundle of S2 , and the first two columns are two-fold covering projections. We remark that we are making here an abuse of notation. The multiplicative group of the unit quaternions is usually denoted by Sp(1) and called the symplectic group; this is the 3-sphere canonically, hence our notation. Now we want to describe the finite subgroups of the 3-sphere. These were known since the 19th century. For instance A. Cayley gave the list of these groups and the list of all the unit quaternions in each (see for instance [223]). Since one has a 2-fold covering S3 → SO(3), which is actually a group homomorphism with kernel ±1, it follows that each finite subgroup Σ of SO(3) lifts to a finite subgroup of S3 doubling its order. The cyclic groups of order r lift to cyclic groups of order 2r and the triangle subgroups Σ(p,q,r) lift to the so-called binary polyhedral groups: (i) the binary dihedral group, of order 4r; (ii) the binary tetrahedral group, of order 24; (iii) the binary octahedral group, of order 48; (iv) the binary icosahedral group, of order 120. These are usually denoted < p, q, r > but to be consistent with our notation in §2 we prefer to denote them Γ(p,q,r) . 3.2 Remark. It is worth making clear that these binary polyhedral groups are not the corresponding full triangle groups Σ∗ of §2 above, though their orders coincide. The binary groups live in S3 while the full triangle groups live in O(3). The groups S3 and O(3) are both double covers of SO(3) and this is why the orders of the groups coincide. However O(3) is a trivial double cover, isomorphic to SO(3) × Z2 , while S3 is simply connected and is therefore the universal cover of SO(3). This means that S3 is also isomorphic to the group Spin(3) (see [124] and Chapter IV below). 3.3 Theorem. The only finite subgroups of the 3-sphere S3 are the double covers of the finite subgroups of SO(3) and the cyclic groups of odd order.

50

Chapter II. On the 3-dimensional Brieskorn Manifolds

The proof of this well-known result is now easy: if a subgroup Γ ⊂ SU (2) contains the centre ±1, then it is the lifting of a subgroup of SO(3) and there is nothing to prove. Thus we only have to prove that if Γ ⊂ SU (2) does not contain the centre, then it is a cyclic group of odd order. For this we observe: (i) if a subgroup Γ ⊂ SU (2) does not contain the centre, then the restriction to π Γ of the projection S3 → SO(3) is an injective morphism and therefore Γ is isomorphic to its image Σ ⊂ SO(3); (ii) if g is an element  in S3 suchthat g 2 = 1, then g = ±1. In fact, think of g as z1 z2 a unitary matrix . Then one has: −¯ z2 z¯1  2

g =

z12 − |z2 |2 z2 (z1 + z¯1 ) −¯ z2 (z1 + z¯1 ) z¯12 − |z2 |2



 1 = 0

 0 ; 1

the two equations given by the anti-diagonal imply that either z2 = 0 or else z1 is purely imaginary, say z1 = iy1 with y ∈ R; if this happens, then z12 = −y12 ≤ 0 and by hypothesis one has z12 − |z2 |2 = 1, which is not possible. Hence one must have z2 = 0 and therefore z1 = ±1. Now, the inverse of an element in S3 is uniquely defined, so the elements in a subgroup Γ ⊂ S3 , others than the identity, necessarily come in pairs. Hence if −1 ∈ / Γ, then the order of Γ is odd, by (ii) above. Thus (i) implies that Γ is isomorphic to a subgroup of SO(3) of odd order, hence Γ is cyclic by Theorem 2.5.

II.4

Brieskorn manifolds and Klein’s theorem

We recall that SU (2) acts linearly on C2 with the origin 0 as the only fix point; the action is free away from 0. Also, this action, being unitary, preserves the usual Hermitian metric on C2 and therefore preserves all the spheres centred at 0. So each orbit, other than 0 itself, is a 3-sphere with centre at 0; the unit sphere is the orbit of (1, 0) ∈ C2 . Each finite subgroup Γ ⊂ SU (2) acts naturally on C2 and on the unit 3-sphere S3 ⊂ C2 . The space of orbits Γ\C2 is a 2-dimensional complex analytic variety VΓ (c.f. [52, 102]) with an isolated singularity at the image of 0, that we denote by ˆ 0. In Chapter I we defined the link of an isolated singularity in an analytic space; this turns out to be the smooth boundary of a regular neighborhood of the singular point. Hence the link of ˆ0 in VΓ is MΓ = Γ\S3 , a smooth compact 3-manifold whose universal cover is S3 and Γ is its fundamental group. Notice that Γ acts naturally on the 3-sphere by left (or right) multiplication in the group. The action we are taking here corresponds to left multiplication (for compatibility with the complex structure). Felix Klein [114] proved in the 19th century a remarkable theorem about these surfaces VΓ . To state his result, let us introduce one of his ideas in this direction, which is to me a keystone in today’s geometric invariant theory (c.f. [178]).

II.4. Brieskorn manifolds and Klein’s theorem

51

Let H d denote the space of homogeneous polynomials of degree d in two complex variables x, y. This is a vector space of dimension d+ 1, with a basis given by the monomials {xd , xd−1 y, . . . , xy d−1 , y d }. The action of SU (2) on C2 defines a linear action of this group on H d in the obvious way: given g ∈ SU (2) and h ∈ H d define the action by (g, h) → hg where hg is defined by hg (x, y) = h(g(x, y)). In this way one has a representation of SU (2) in H d . [In fact these are all the irreducible representations of SU (2).] Of course that if we are given a subgroup Γ ⊂ SU (2), one can restrict this action to Γ. 4.1 Definition. An element h ∈ H d is Γ-invariant if h(g(x, y)) = h(x, y) for all g ∈ Γ and all (x, y) ∈ C2 . It is clear that the set of all Γ-invariant polynomials of degree d is a complex vector space HΓd . Moreover, given h1 ∈ HΓd1 and h2 ∈ HΓd2 their product is in HΓd1 +d2 . Hence the set of all Γ-invariant polynomials of all degrees forms a graded algebra, that we denote by HΓ∗ , which as a vector space is the direct sum of the HΓd . The theorem of Klein that we are talking about can be phrased as follows. 4.2 Theorem (Klein). Let Γ be a finite subgroup of SU (2). Then the algebra HΓ∗ of Γ-invariant polynomials is generated by three homogeneous polynomials, say h1 , h2 , h3 , of various degrees, which satisfy a (essentially single) polynomial equation f (h1 , h2 , h3 ) = 0 , where f is weighted homogeneous. These three polynomials define a Γ-invariant function H : C2 → C3 , which induces a homeomorphism between the orbit space VΓ = Γ\C2 and the surface V = f −1 (0). This map is actually a diffeomorphism from VΓ − ˆ 0 into V − 0 and carries MΓ = (Γ\SU (2)) into the link of 0 in V , which is V ∩ S5 , where S5 is the unit sphere in C3 . In fact Klein finds the explicit polynomials that generate these algebras for each of the finite subgroups of SU (2). For instance, if Γ = Zr is cyclic of order r, then the generators of the algebra HZ∗r can be taken to be: h1 = xr + y r

,

h2 = ixr − iy r

and h3 = 2ixy ,

which clearly satisfy: h21 + h22 + hr3 = 0 and the surface VZr = Zr \C2 is the hypersurface V in C3 defined by V = { z12 + z22 + z3r = 0 } . So the link of V is the lens space Zr \S3 . In particular, for r = 2 this is RP 3 = SO(3). We refer to Klein’s book for the proof of Theorem 4.2. Here we follow [171] and prove a slight refinement of Klein’s theorem, which leaves out a few cases

52

Chapter II. On the 3-dimensional Brieskorn Manifolds

but is more precise in its statement and has the advantage of motivating Milnor’s proof of his theorem for the Brieskorn manifolds in the hyperbolic case, which we discuss in Section 6 below. 4.3 Theorem. Let Γ = Γ(p,q,r) be a binary triangle subgroup of SU (2) and let Π = ∗ Π(p,q,r) be its commutator, Π = [Γ, Γ]. Then the algebra HΠ of Π-invariant polynomials is generated by three polynomials h1 , h2 , h3 of orders k/p, k/q, k/r , respectively, where k is the order of the triangle subgroup Σ(p,q,r) ⊂ SO(3), Σ(p,q,r) = Γ(p,q,r) / ± 1, which satisfy the (essentially single) polynomial relation: hp1 + hq2 + hr3 = 0. The corresponding Π-invariant map C2 → C3 determines a homeomorphism between the space of orbits Π\C2 and the Brieskorn variety: V(p,q,r) = {(z1 , z2 , z3 ) ∈ C3 | z1p + z2q + z3r = 0 } . This homeomorphism is actually a diffeomorphism away from the singular point in each surface, and it induces a diffeomorphism between the corresponding links. The statement “essentially single” in Theorem 4.3 means that the ideal of all polynomial relations among these polynomials is generated by the aforementioned relation. Also, we recall that the order of Σ(p,q,r) is half the order of Γ(p,q,r) , which equals the order of the full triangle group Σ∗(p,q,r) . Hence (2.3) yields that the order of Σ(p,q,r) is:

Σ(p,q,r) =

2 . ( p−1 + q −1 + r−1 ) − 1

Let us make some comments before coming to the proof of Theorem 4.3. First we remark that the equivalence in Theorem 4.3 is in fact an isomorphism of analytic spaces, but we will not discuss that here. In fact these singularities are the prototype of what is called a “taut singularity” (see [121]). These are, by definition, isolated surface singularities whose topology determines uniquely the analytic structure. This is not always so. For instance, it follows from [171, Theorem 7.3] that the Brieskorn manifolds M(2,7,14) and M(3,4,12) are both circle bundles with Euler class −1 over a Riemann surface of genus 3. Hence the corresponding surfaces are homeomorphic. However (c.f.[168, Th. 9.1]) the Milnor number of the first surface is 78, while for the latter this number is 66, so they are not analytically equivalent. Similar remarks apply to many other Brieskorn manifolds, as for instance M(2,9,18) and M(3,5,15) . It is an exercise to show that the binary icosahedral group Γ(2,3,5) is a perfect group, i.e., it equals its own commutator. The commutator of Γ(2,3,4) is Γ(2,3,3) , the commutator of Γ(2,3,3) is Γ(2,2,2) , which is the so-called quaternion group, consisting of the quaternions ±1, ±i, ±j, ±k. The commutator of the dihedral group Γ(2,2,r) is the cyclic group Zr .

II.4. Brieskorn manifolds and Klein’s theorem

53

Hence Theorem 4.3 gives the equivalences: iv) V(2,2,r) ∼ = (Zr \C2 ), (iii) V(2,3,3) ∼ = (Γ(2,2,2) \C2 ), (ii) (i)

V(2,3,4) ∼ = (Γ(2,3,3) \C2 ), V(2,3,5) ∼ = (Γ(2,3,5) \C2 ).

In this list there are missing the cases (Γ(2,3,4) \C2 ) and (Γ(2,2,r) \C2 ) , r > 2. Klein’s theorem is also true for these, and the corresponding singular surfaces are: z12 + z23 + z2 z33 = 0 for the binary octahedral group Γ(2,3,4) , and z12 + z22 z3 + z3r+1 = 0 for the binary dihedral group with 4r elements. In the special case r = 2 this surface is equivalent to V(2,3,3) . We also notice that the Brieskorn manifold M(2,3,5) ∼ = Γ(2,3,5) \S3 is a homology sphere, since the group Γ(2,3,5) is perfect. This is the famous Poincar´e’s homology sphere. Notice that examples like this may not occur in higher dimensions, as pointed out in §8 of the first chapter. The point is that when the link of a hypersurface singularity has dimension more than 3, then it is highly connected and therefore it is a homology sphere iff it is a homotopy sphere, and in this case Smale’s h-cobordism theorem implies that the manifold is actually a topological sphere. The singularities constructed above play a very special role in geometry and the theory of singularities, and they receive different names according to the property one is looking at. They are called, for instance, Klein singularities, Du Val singularities or rational double points. They are also the simple (or 0-modal) singularities in Arnold’s notation (see for instance [9]). We refer to [65] for a (yet not exhaustive) list of 15 characterizations of these singularities. Klein’s theorem gives two of them. These groups can also be regarded as Coxeter groups and there are very interesting results in this respect, see for instance [42, 45]. Proof of 4.3. This theorem is carefully proved in Milnor’s article [171], so we only indicate here the main ideas. We recall that a character of a group Γ means a 1-dimensional unitary representation of Γ, i.e., a homomorphism χ : Γ → U (1) ⊂ C∗ := C − 0. Given Γ ⊂ SU (2) and a character χ, let HΓd,χ denote the set of all homogeneous polynomials of degree d which are χ-invariant, i.e., they transform according to the rule: h(g(x, y)) = χ(g)h(x, y)

54

Chapter II. On the 3-dimensional Brieskorn Manifolds

so they are “invariant up to multiplication by χ”. [These polynomials were called “invariant forms” by Klein in [114].] Since χ is a homomorphism, the set HΓd,χ is a complex vector space. Furthermore, given characters χ1 , χ2 , and polynomials d1 +d2 ,χ1 χ2 h1 ∈ HΓd1 ,χ1 and h2 ∈ HΓd2 ,χ2 , the product h1 · h2 is in HΓ , where the product of the characters is defined in the obvious way. Hence these polynomials form a bigraded algebra that we denote HΓ∗,∗ . Notice that HΓ∗,1 is by definition the set of Γ-invariant polynomials. Given Γ ⊂ SU (2) and h ∈ HΓd,χ for some character χ, for every g = xyx−1 y −1 d in the commutator Π of Γ, one has χ(g) = 1. Thus one has an inclusion HΓd,χ ⊂ HΠ . On the other hand, Π is a normal subgroup of Γ, hence the quotient group Γ/Π acts d , which is therefore a representation space for this group. Since the linearly on HΠ group Γ/Π is finite and Abelian, it follows (see [171, 4.1]) that the representation d space HΠ splits as the direct sum of the HΓd,χ as χ varies over the characters of Γ. Hence one has an isomorphism of algebras:  ∗,∗ ∗ = HΓ . (4.4) HΠ Now consider a polynomial h ∈ HΓd,χ , for some d and some χ. Since h is homogeneous of degree d, it follows that h must vanish over d lines (maybe not necessarily different) 1 , . . . , d through the origin in C2 . These lines are necessarily permuted by the elements of Γ, and the linear equations of 1 , . . . , d determine h uniquely, up to multiplication by a constant. Conversely, given d points in CP 1 which are permuted by the action of Γ, the corresponding lines in C2 determine a homogeneous polynomial h of degree d, which satisfies that for each γ ∈ Γ, the rotated polynomial f (g(x, y)) is a scalar multiple of f (x, y). Thus we can define a character of Γ by: χ(g) = f (g(x, y))/f (x, y). By construction one has that h is in HΓd,χ . Now suppose Γ is a triangle subgroup Γ(p,q,r) of SU (2). Then its action on CP 1 ∼ = S2 factors through an action of the triangle group Σ(p,q,r) ⊂ SO(3), because the element −1 carries each line into itself. Now consider the triangle T(p,q,r) ⊂ S2 whose edges give rise to the full triangle group Σ∗(p,q,r) , and let T be the triangle obtained by reflecting T(p,q,r) in one of its edges. The generators of Σ(p,q,r) are rotations around the vertices v1 , v2 , v3 of T(p,q,r) by angles 2π/p, 2π/q, 2π/r, and T is a fundamental domain for the action of Σ(p,q,r) , as we already know from Corollary 2.4 or from the discussion in §1. One has three special orbits, which correspond to the three vertices v1 , v2 , v3 of T, and all other orbits have as many points as there are copies of the triangle T in S2 , i.e., the order k of Σ(p,q,r) . Since the stabilizers of the vertices are cyclic groups of orders p, q, r, respectively, it follows that the orbits of the three vertices have k/p, k/q, k/r points. These three special orbits determine as above three k

polynomials h1 ∈ HΓp

,χ1

k

, h2 ∈ HΓq

,χ2

k

, h3 ∈ HΓr

,χ3

. [It is worth saying that if


II.5. The universal cover SL(2, R)

55

we think of the triangle groups as being groups of symmetries of polyhedra, then the three special orbits correspond to: (i) the vertices of the polyhedron; (ii) the middle points of the edges; and (iii) the middle points of the faces.] The claim now is that these polynomials h1 , h2 , h3 generate the bigraded ∗ algebra HΓ∗,∗ , so they generate HΠ and they satisfy the polynomial relation of Theorem 4.3 above. (This is also Lemma 4.3 in [171].) Since these polynomials are Π-invariant, they define a map φ : Π\C2 −→ C3 , whose image is contained in the Brieskorn variety V(p,q,r) . We claim that this map is injective. For this we consider points (x1 , y1 ) and (x2 , y2 ) in C2 which are not in the same Π-orbit. Denote by {π1 , . . . πm } the elements of Π, and choose some polynomial g(x, y) (maybe not homogeneous) which vanishes on (x1 , y1 ) but does not vanish at any point in the Π-orbit of (x2 , y2 ). Now define a Π-invariant polynomial h by: h(x, y) = g(π1 (x, y)) g(π2 (x, y)) · · · g(πm (x, y)) . By construction one has h(x1 , y1 ) = h(x2 , y2 ). Now we express h as the sum of homogeneous polynomials of various degrees. Since h1 , h2 and h3 generate the bigraded algebra HΓ∗,∗ , we must have that one of these satisfies that hi (x1 , y1 ) = hi (x2 , y2 ). Hence the map φ is injective. The rest now follows from general arguments, using the local conical structure of V(p,q,r) and the fact that away from the isolated singular point in each surface, both VΓ = Π\C2 and V(p,q,r) are complex manifolds of the same dimension (c.f. [171, §4] for details).

II.5

The group P SL(2, R) and its universal
R) cover SL(2,

Let us look now at the group P SL(2, R) of orientation preserving isometries of the hyperbolic plane H2 , introduced in §2 above. We recall that it acts on H2 with isotropy SO(2), hence it is diffeomorphic to the open solid torus S1 × H2 . Given points z1 , z2 ∈ H and unit tangent vectors v1 , v2 based at z1 , z2 , there is one and only one element in P SL(2, R) taking z1 to z2 and whose derivative carries v1 into v2 . For the rest of this chapter it is actually better to work with the Lobachevsky model for hyperbolic geometry, so we think of H2 as being the upper half-plane Im z > 0 of the complex plane C. For simplicity we set H = H2 . We  identify  a b az+b P SL(2, R) with the group of all M¨ obius transformations z → cz+d , where c d is an element in SL(2, R) and two such matrices define the same M¨obius transformations iff they differ by sign.

56

Chapter II. On the 3-dimensional Brieskorn Manifolds

Notice that the action of P SL(2, R) on the tangent bundle T H ∼ = H × C is via the derivative:   dg g · (z, w) = g(z) , (z) · w , dz and this action is free away from the zero-section, because if an element g ∈ P SL(2, R) fixes a point z ∈ H, then it is locally a rotation (more precisely it is an “elliptic element” in P SL(2, R), using the standard terminology). It follows that given any point (z, v) ∈ T H with |v| =  0, the orbit of this point is a copy of P SL(2, R). In particular the orbit of the point (i, 1) embeds P SL(2, R) in T H as the unit tangent bundle of the hyperbolic plane. Since P SL(2, R) is topologically a solid torus S1 × H2 , its fundamental group
is isomorphic to Z and its universal cover SL(2, R) is diffeomorphic to R×H2 ∼ = R3 ,
but its Lie group structure is quite interesting. This group SL(2, R) is a central extension of P SL(2, R) and one has a short exact sequence:
0 −→ Z −→ SL(2, R) −→ P SL(2, R) −→ 0 ,
where the kernel Z of the projection SL(2, R) −→ P SL(2, R) is the centre of
SL(2, R). We need in the sequel a description of this group due to Milnor [171, §5], which is not standard. For this we need to introduce the differential forms of fractional degree, first considered by H. Petersson (1930). It is classical in the theory of Riemann surfaces to consider Abelian differentials, which are expressions of the form f (z) dz with f holomorphic, as well as quadratic differentials, i.e., expressions of the form f (z) dz 2 . More generally, for any integer k ≥ 0, a differential form of degree k on H (or on an open set of C) can be defined as a complex-valued holomorphic function of two variables, of the form φ(z, w) = f (z) wk , where z varies over H and w (= dz) varies over C. These can be regarded as functions defined on the holomorphic tangent bundle T H ∼ = H × C. Given such a differential form φ(z, w) on H and an element g ∈ P SL(2, R), the pull-back g ∗ φ is the form   k  dg dg (z) · w = f (g(z)) wk . g φ(z, w) = φ g(z) , dz dz ∗

This allows us to speak of differential forms (of any integral degree) invariant under the action of a certain subgroup of P SL(2, R), these are called automorphic forms and we will come to them in §6 below. We need to generalize these concepts (following [171]), replacing the integer k by an arbitrary rational number. For this we make the convention that the

II.6. The Brieskorn manifolds, hyperbolic case

57

 ∗ of the non-zero complex variable w is to vary over the universal covering group C numbers, which is of course equivalent to the additive group of C; the isomorphism ∗ e : C −→ C of complex Lie groups is obtained by lifting the exponential map e(z) = e2πiz from C to C∗ . The kernel of the projection is isomorphic to Z and it is generated by the image of e(1). 5.1 Definition. (Milnor) A differential form of (fractional) degree α ∈ Q on the  ∗ of the form: upper half-plane H is a holomorphic function on H × C φ(z, w) = f (z) wα , where f is a holomorphic function on H and w varies over the universal covering group of C∗ , the non-zero complex numbers. Here it is understood that wα is to  ∗ and then projected to C∗ to be multiplied by f (z). be evaluated in C 5.2 Definition. (Milnor) A labeled holomorphic map on H means a holomorphic function g : H → H with nowhere-vanishing derivative, together with a lifting of  ∗ (a “labeling”). the derivative from C∗ to C As noted by Milnor ([171, p. 201]) one has: 5.3 Proposition. The set of all labeled biholomorphic maps from H to itself forms a
group, which is isomorphic to SL(2,R), the universal covering group of P SL(2,R).  ∗ by: This group acts on H × C  
dg g · (s, w) = g(z) , (z) · w , dz
where SL(2, R) is acting on H by projecting it to SL(2, R). This action is free
and therefore each orbit is a copy of SL(2, R) embedded in the complex manifold ∗  H×C .

II.6

Milnor’s theorem for the 3-dimensional Brieskorn manifolds. The hyperbolic case

In this section we state the following theorem of Milnor [171] and give an outline of its proof, which is formally similar to the proof of Theorem 4.3. We refer to Milnor’s article for the complete proof.  be its 6.1 Theorem. Let Γ = Γ(p,q,r) be a triangle subgroup of P SL(2, R), let Γ

lifting to the universal covering group SL(2, R), and let Π ⊂ SL(2, R) be the  Then the orbit space (Π\  SL(2,
commutator subgroup of Γ. R)) is diffeomorphic to the 3-dimensional Brieskorn manifold M(p,q,r) .

58

Chapter II. On the 3-dimensional Brieskorn Manifolds

Before coming to the proof of this theorem, let us remark that when we lift  of SL(2,
R), the lifting necessarily contains Γ(p,q,r) ⊂ P SL(2, R) to a subgroup Γ ∼
the centre C = Z of SL(2, R), which is the kernel of the projection into P SL(2, R).  SL(2,
Hence the quotient spaces (Γ\P SL(2, R)) and (Γ\ R)) are obviously diffeomorphic 3-manifolds, and they are closed (i.e., compact and with empty boundary) because (Γ\P SL(2, R)) is a Seifert manifold (see III.1 below) that fibres with three exceptional fibres over the orbifold Γ\H, which is the sphere S2 with three marked  points. However, in Theorem 6.1 we are considering the commutator subgroup Π, and there is no reason why this group should contain the centre C, and in fact it does not in general. Hence the Brieskorn manifold M(p,q,r) will not be in general of the form (G\P SL(2, R)) for a subgroup of P SL(2, R) (c.f. Chapter III). An exception is when the triple (p, q, r) is (2, 3, 7). The corresponding triangle  (2,3,7) ⊂ SL(2,
group is perfect and its lifting Γ R)) is also perfect. [In fact a simple computation shows that this is the only triangle subgroup of P SL(2, R) which is perfect and its lifting to the universal cover is also perfect; it would be interesting to understand why this happens.] Milnor’s proof of Theorem 6.1 is along the same lines as his proof of Theorem 4.3, with automorphic forms replacing the homogeneous polynomials. We recall (see for instance [30]) that the classical automorphic functions were introduced by Poincar´e in the 1880s as holomorphic (or meromorphic) functions on H satisfying certain conditions with respect to a discrete subgroup of M¨obius transformations. For Theorem 6.1 we need to consider what Milnor calls automorphic forms of fractional degree. For this we consider differential forms of fractional degree as in the previous section. These are expressions of the form φ(z, w) = f (z)wα , where f (z) is a holomorphic function on H, and w varies over the universal covering group of C∗ , the non-zero complex numbers (we recall that wα is to be evaluated  ∗ and then projected to C∗ to be multiplied by f (z)). in C
Given a differential φ(z, w) = f (z)wα on H and an element g ∈ SL(2, R), its pull-back g ∗ φ is:  
dg (z) · w . g ∗ φ(z, w) = φ g(z) , dz
6.2 Definition. Let Γ be a discrete subgroup of SL(2, R) and let χ : Γ → U (1) ⊂ C∗ α be a character. A differential φ(z, w) = f (z)w on H is χ-automorphic if g ∗ (φ) = χ(g) · φ for every g ∈ Γ. If χ ≡ 1 is the constant character, so that g ∗ (φ) = φ for all g ∈ Γ, then φ(z, w) is said to be Γ–automorphic. Notice that such a form vanishes at a point x (to order s) if g vanishes at x (to order s), and in this case g vanishes in the whole orbit of x, since the form is automorphic.

II.6. The Brieskorn manifolds, hyperbolic case

59

Let us denote by Aα,χ the complex vector space of χ-automorphic forms of Γ degree α. We denote simply by Aα Γ those corresponding to the constant character 1, i.e., the Γ-automorphic forms. We have, just as for the homogeneous polynomials of Section 4, that each of these is a vector space. Taking all of them, of all degrees and for all characters, we get the bigraded algebras A∗Γ

and A∗,∗ Γ

of automorphic and χ-automorphic forms, respectively. The first step for proving Theorem 6.1 is to show that given the extended  one has that each vector space =Γ  (p,q,r) and its commutator Π, triangle group Γ α,χ α  as χ varies over the characters of Γ. AΠ  splits as direct sum of its subspaces AΓ  ∗  Thus one has that the graded algebra A of Π-automorphic forms is the direct sum  Π

of the bigraded algebras A∗,∗  . This is easy and similar to the previous assertion Γ for polynomials and subgroups of SU (2). The aim is to show that the algebra A∗Π  is generated by three automorphic forms which satisfy the relation given by the corresponding Brieskorn polynomial.  (p,q,r) has a presentation with For this Milnor shows (Lemma 3.1) that the group Γ generators γ1 , γ2 , γ3 , which represent rotations through the three vertices of the triangle T(p,q,r) and satisfy the relations: γ1p = γ2q = γ3r = γ1 γ2 γ3 . Letting k be the rational number defined by, k =

1 , 1 − p−1 − q −1 − r−1

so that π/k is the area of the triangle T(p,q,r) , Milnor constructs a character χ0 of  defining it on the generators by: Γ χ0 (γ1 ) = e2πik/p

,

χ0 (γ2 ) = e2πik/q

,

χ0 (γ3 ) = e2πik/r ,

and shows that if an automorphic form φ ∈ Aα,χ does not vanish at any vertex of  Γ α/k

T(p,q,r) , then its degree α is a multiple of k and the character χ is χ0 . [Recall that in the spherical case k is the order of the triangle group in question, hence the order of the generic orbits.] To construct the generators of the algebra of Π-invariant automorphic forms, one may recall that in the spherical case this was done by looking at the orbit of each vertex of T ; such orbit defines a set of lines through the origin in C2 which are Γ-invariant and give rise to a homogeneous polynomial hi (well defined up to a constant) and a character χi , defined ad hoc so that h is χi -invariant. In 0 the hyperbolic case the key is (Lemma 6.3 in [171]) proving that the space Ak,χ  Γ contains one and (up to a constant multiple) only one automorphic form which

60

Chapter II. On the 3-dimensional Brieskorn Manifolds

vanishes at any given point of H (and therefore at the orbit of the point, since it is automorphic). If the chosen point is a vertex of T , say v1 , then the form one gets must have a p-fold holomorphic root φ1 , and Milnor shows that this must be also an automorphic form for some character χ1 satisfying χp1 = χ0 (Lemma 5.3 in [171]). Hence, just as for polynomials, the orbit in H of each vertex defines an automorphic form of Γ, for some character. In this way Milnor arrives at his main Lemma (6.4), that given the extended  the bigraded algebra A∗,∗ is  = Γ  (p,q,r) and its commutator Π, triangle group Γ  Γ ∗ isomorphic to the graded algebra AΠ and is generated by three forms k/p,χ1 Γ

φ1 ∈ A

,

k/q,χ2 Γ

φ2 ∈ A

,

k/r,χ3 Γ

φ3 ∈ A

,

where the characters satisfy the relation χp1 = χq2 = χr3 = χ0 .  Furthermore, the automorphic form φi has a simple zero at each point of the Γorbit of the vertex vi and no other zeroes, and they satisfy the polynomial relation: φp1 + φq2 + φr3 = 0. The three forms φ1 , φ2 , φ3 are complex-valued holomorphic functions on  ∗ and they are never simultaneously zero, so they define a holomorphic map: H×C  ∗ −→ C3 Φ = (φ1 , φ2 , φ3 ) : h × C whose image is contained in the complex manifold V(p,q,r) − 0, obtained by removing the singular point from the corresponding Brieskorn variety. As we know from  ∗ by holomorphic transforma
Section 5, the group SL(2, R) acts freely on H × C  ∗ is foliated by the orbits of SL(2,
tions. Hence H × C R), which are all copies of ∗   this group. Thus the quotient of H× C by Π is a (non-compact) complex manifold  SL(2,
of dimension 2, which is foliated by manifolds of the form (Π\ R)). One can show that Φ is actually a biholomorphism and induces a diffeomorphism between  SL(2,
one of the orbits (Π\ R)) (anyone you prefer) and the Brieskorn manifold
M(p,q,r) .

II.7

Brieskorn-Hamm complete intersections. The theorem of Neumann

Consider now a Pham-Brieskorn polynomial in n complex variables, f (z) = z1a1 + · · · + znan , ai ≥ 2 , n ≥ 3 . Given any n-tuple α1 = α11 , . . . , α1n of non-zero complex numbers, the variety: Vα1 = { α11 z1a1 + · · · + α1n znan = 0 }

II.7. Brieskorn-Hamm complete intersections

61

has an isolated singularity at 0 and it is equivalent to a Brieskorn variety. However the way this is placed in Cn depends on the coefficients αij . Suppose you have another vector of coefficients α2 = α21 , . . . , α2n , then one has another equivalent Brieskorn variety Vα2 . It is an exercise to show that if all the (2 × 2)-minors of the (2 × 2n) matrix whose rows are α1 , α2 have non-zero determinant, then these two varieties intersect transversally away from 0 and define a complete intersection singularity, of codimension 2, with an isolated singularity at  0. More  generally, H. Hamm [96] showed that given any (n − 2) × n matrix A = αi,j the corresponding n − 2 varieties define a 2-dimensional complete intersection VA with an isolated singularity at 0 ∈ Cn if and only if every (n − 2) × (n − 2) subdeterminant is non-zero. In this case the link, M = VA ∩ S2n−1 , is a smooth 3-manifold whose diffeomorphism type depends only on the n-tuple a1 , . . . , an [96]. Following Hamm and [183], we call VA a Brieskorn-Hamm complete intersection. If n = 3 this is a Brieskorn variety as before. Now consider a convex n-sided polygon ∆ ⊂ H, bounded by geodesics, whose interior angles are π/a1 , . . . , π/an , let Σ∗ be the group generated by the reflections (inversions) σ1 , . . . , σn in the edges of ∆, and let Σ ⊂ P SL(2, R) be the group of orientation preserving isometries of H generated by reflections on the edges of ∆. These groups are called polygonal, for obvious reasons. They were considered by W. Dyck (1882) and one can show, just as for the triangle groups, that Σ∗ has a presentation with generators σ1 , . . . , σn and relations σi2 = (σi σi+1 )ai = 1 for all i modulo n. The group Σ is generated by rotations around the vertices by twice the corresponding angles, and it has a presentation: τ1 , . . . , τn | τ1a1 = · · · = τnan = τ1 · · · τn = 1 .
Following [183] we lift Σ to the universal covering SL(2, R). Its lifting Γ = Γ(a1 ,...,an ) has a presentation γ1 , . . . , γn , c | γ1a1 = · · · = γnan = c , γ1 · · · γn = cn−2  . (Notice that if n = 2 nothing changes if we drop the generator c.) Now let
Π ⊂ SL(2, R) be the commutator subgroup of Γ. Just as for the triangle groups,  ∗ and one has the corresponding one has that both Γ and Π act (freely) on H × C algebras of automorphic and χ-automorphic forms of fractional degree, for the various characters of Γ. Again, since Π\Γ is finite and Abelian, one has an iso∗,∗ morphism of algebras A∗Π ∼ = AΓ and W. Neumann proved in [183] that these k/a ,χ algebras are generated  by n forms φj ∈ AΓ j j , where k is given by the area a−1 and the characters χ1 , . . . , χn are defined on the of ∆: k −1 = (n − 2) − i generators of Γ by: χj (γi ) = e2πik/ai aj

for j = i ;

and 2

χj (γi ) = e2πik/(ai +1)/aj

for j = i .

62

Chapter II. On the 3-dimensional Brieskorn Manifolds

Each form φj has a simple zero at the vertex vj of ∆ and is uniquely determined by its zeroes up to a multiple. These n forms satisfy polynomial relations: αi1 φa1 1 + · · · + αin φann = 0 ; i = 1, . . . , n − 2 ,   such that the matrix A = αij has all (n − 2) × (n − 2) minors with non-zero determinant. In this way one has the Brieskorn-Hamm complete intersection VA determined by the matrix A and the angles of the polyhedron ∆. We arrive at the following beautiful generalization of Milnor’s theorem. 7.1 Theorem (Neumann). These n forms define a map  ∗ −→ Cn Φ = (φ1 , . . . , φn ) : H × C which is Π-invariant and defines a biholomorphism between the space of orbits  ∗ ) and the non-singular part V ∗ = VA − 0 of the Brieskorn-Hamm Π\(H × C A complete intersection VA =

n−2 

{ αi1 z1a1 + · · · + αin znan = 0 } .

i=1


This map induces a diffeomorphism between Π\SL(2, R) and the link of 0 in VA . In Chapter III we speak more about this theorem and improvements of it made by Neumann himself and by I. Dolgachev.

II.8

Remarks

To complete the story about the relation between groups of isometries in plane geometry and Brieskorn singularities, it remains to consider: (i) What about the Brieskorn singularities { z1p +z2q +z3r = 0} when 1p + 1q + 1r = 1 ? (ii) What about the subgroups of P SL(2, R), other than the triangle groups? And what about the quotients of P SL(2, R) by the triangle groups themselves? (not going to the universal cover and then taking the commutator subgroup there). (iii) What about the groups of isometries of the Euclidean plane? These three questions are answered in Chapter III. The answer to (i) was given by Milnor himself in [171], showing that these singularities correspond to quotients related to the nilpotent group N of real 3 × 3 upper triangular matrices. The Brieskorn singularities correspond to three of the discrete subgroups of N ; this study was completed in [59] and [183]. A subgroup Γ of P SL(2, R) which is discrete is called a Fuchsian group, and it is said to be cocompact if the orbit space Γ\P SL(2, R) is compact. These are the groups relevant for question (ii), since the link of an isolated complex

II.8. Remarks

63

surface singularity is a compact 3-manifold. Dolgachev gave in [57, 58] a beautiful construction of surface singularities using cocompact Fuchsian groups, which is very closely related to Milnor’s work for the Brieskorn manifolds. This provides, for every cocompact Fuchsian group, an isolated surface singularity whose link is Γ\P SL(2, R). This was extended in [59] and [183] to all discrete subgroups of
SL(2, R). Question (iii) was considered in [231] and we discuss it in Chapter III below.

Chapter III

3-dimensional Lie Groups and Surface Singularities The theorems of Klein, Milnor and Neumann discussed in Chapter II, together with very deep and important results by Hirzebruch, Dolgachev, Neumann himself and others that I have not mentioned yet, give us a very good understanding of the relation between 3-dimensional Lie groups and complex surface singularities; this is the topic that we explore in this chapter. Our basic references here are [207, 59, 194, 204, 104, 186, 231]. It turns out that up to isomorphism, there are six different 3-dimensional Lie groups that admit discrete subgroups with compact quotient (we call these uniform subgroups, a term frequently used in the literature). In five of these cases the orbit spaces one gets are Seifert manifolds, and these are strongly related with the so-called quasi-homogeneous surface singularities. The remaining case gives rise to the cusp-singularities, whose links are torus bundles over the circle. In this chapter we explain these relationships. We begin by describing a few well-known facts about quasi-homogeneous singularities and Seifert manifolds, then we describe the 3-dimensional Lie groups with uniform subgroups and their orbit spaces. Their relation with singularities is discussed case by case, including two cases which apparently do not come into the scene and are usually left aside  + (2) and when talking about complex singularities (these are the solvable group E 3 the Abelian group R ). We show that there are several interesting properties in common for all six cases. We also discuss a relationship between the property of being Gorenstein for the complex surfaces in question and the Lie algebras of the corresponding groups.

III.1

Quasi-Homogeneous surface singularities

The quasi-homogeneous singularities play a very distinguished role in singularity theory. Their geometry and structure has been widely studied by many authors,

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most notably by H. Pinkham, P. Orlik and Ph. Wagreich, I. Dolgachev and W. Neumann. We refer to [256] for a beautiful introduction to the subject. We recall that a polynomial h in n complex variables is weighted homogeneous if there exist non-zero integers (q1 , . . . , qn ) and a positive integer d such that: h(tq1 z1 , . . . , tqn zn ) = td h(z1 , . . . , zn ) . Equivalently, we demand that there exist non-zero rational numbers (w1 , . . . , wn ), called the weights of h, for which h is a sum of monomials z1α1 · · · znαn such that αn α1 + ··· + = 1. w1 wn Given these weights we recover d as being the smallest integer such that for each i = 1, . . . , n there exists an integer qi so that qi wi = d. These are the qi . For example, if h is a Pham-Brieskorn polynomial, h(z1 , . . . , zn ) = z1a1 + · · · + znan , then the weights are obviously (a1 , . . . , an ). For the polynomial f (z1 , z2 ) = z1a1 + z1 z2a2 , a1 > 1, a2 ≥ 1 , the weights are: w1 = a1 and w2 = a1 a2 /(a1 − 1). In this case one has q1 = a2 , q2 = a1 − 1 and d = a1 a2 . One has in this case: f (ta2 z1 , ta1 −1 z2 ) = ta1 a2 f (z1 , z2 ) , as it should be. Now let V ⊂ Cn be an algebraic variety defined by homogeneous polynomials h1 , . . . , hn with same weights. Let q1 , . . . , qn be the corresponding exponents, defined as above. One can define an action of C∗ on Cn , σ : C∗ × Cn −→ Cn by σ(t, (z1 , . . . , zn )) = (tq1 z1 , . . . , tqn zn ). Notice this is exactly the action considered in I.1 above for the Brieskorn varieties. Just as in that case, if the exponents qi are all > 0, then the restriction of this action to the positive real numbers R+ gives a real analytic flow, whose orbits are transversal to all the spheres in Cn centred at 0, and they converge to the origin. Hence every orbit contains 0 in its closure. Conversely, if a complex analytic variety V ⊂ Cn admits such a C∗ -action, then V is globally defined and it is (globally) a cone over its intersection with the unit sphere. Furthermore, by [194, 1.1.2] one also has that V is actually algebraic and the ideal of polynomials (in n variables) vanishing on V is generated by weighted homogeneous polynomials. In fact one has even more (see [194, 1.1.3]): if V ⊂ Cn is an algebraic variety and there is a C∗ -action on V , C∗ × V → V , given by a morphism of algebraic varieties, then:

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67

(i) there is an embedding of V in some Cm and a C∗ -action on Cm for which V is an invariant set and the induced action on V coincides with the given one; and (ii) one can choose the coordinates suitably, so that the action takes the form: σ(t, (z1 , . . . , zm )) = (tq1 z1 , . . . , tqm zm ) for some qi ∈ Z. This means that the fact that an analytic germ (V, 0) admits a C∗ -action is something intrinsic of the germ, independent of the equations used to define it; and V is automatically algebraic. 1.1 Definition. By a quasi-homogeneous surface singularity we mean a 2-dimensional complex analytic variety V in some affine space Cn with an isolated singularity at 0, together with a good C∗ -action, where good means that 0 is in the closure of every C∗ -orbit. As before, by restricting the action to the positive real numbers R+ ⊂ C∗ one gets a real analytic flow on V , with a fixed point at 0, whose orbits are rays emanating from the origin, transversal to all the spheres around 0, and the orbit space (V −0)/R+ can be identified with the link M , which is automatically a Seifert manifold, with a Seifert decomposition given by the S1 -orbits of the restriction to S1 of the C∗ -action. We recall that a Seifert manifold can be defined to be a 3-manifold M , that we assume to be closed, connected and orientable, endowed with a foliation by oriented circles, such that every fibre has a neighborhood which is diffeomorphic to a torus S1 × D2 and is a union of orbits. (See for instance [206, 193, 110] for details). We can actually assume that this foliation on M is given by an S1 action. One has that there are a finite number of special orbits O1 , . . . , Or and all other orbits are principal. The principal orbits are those where the S1 -action is free. Each such orbit has a neighborhood which is a solid torus T, a union of principal orbits. The special (exceptional) orbits are where the action has nontrivial isotropy. It is easy to see that the isotropies are necessarily finite and cyclic, and therefore the quotient space of M by this action is a closed 2-dimensional manifold S; one has a projection p : M → S, which is a fibre bundle away from the exceptional fibres. Thus we speak of the Seifert fibres of the corresponding Seifert fibration of M . To each exceptional Seifert fibre one attaches a pair (α, β) of normalized (or reduced) Seifert invariants (0 < β < α). These integers satisfy the equation αa + βb = 0 in H1 (N, Z), where N is a small tubular neighborhood of the corresponding exceptional fibre, saturated with Seifert fibres, b is an oriented Seifert fibre on ∂N and a is an oriented curve on ∂N such that the intersection a · b is +1 on ∂N , oriented as the boundary of N . Seifert manifolds are characterized, up to equivariant diffeomorphism, by their Seifert invariants (see [193, 1.10]). These are the normalized orbit invariants (αi , βi ), i = 1, . . . , r , of the exceptional fibres and (in our case) two other invariants

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that we now describe. One is the genus of the 2-dimensional manifold S. It can happen that S is non-orientable, and this has to be taken into account, but that will not happen for the cases that concern us in this text. The other relevant invariant is usually denoted by e; when there are no special fibres, so that M is an S1 -bundle over S, e is the usual Euler number (see [242]). In the general case one must take the special fibres into account. To define it we recall that if E is an oriented S1 bundle over an oriented 2-dimensional, compact, connected, manifold B, then its usual Euler class is the primary (i.e., non-automatically zero) obstruction for constructing a section of E. This class lives in H 2 (B; Z) and it becomes a number when we evaluate it on the orientation class of B. If B has non-empty boundary, then H 2 (B; Z) ∼ = 0, so the bundle is trivial. However, if we fix a choice of a trivialization of E over ∂B, i.e., a section of τ : ∂B → E, then one has an Euler class of E relative to τ , e(E; τ ) ∈ H 2 (B, ∂B; Z) ∼ = Z; evaluating e(E; τ ) on the orientation cycle of the pair (B, ∂B) we obtain an integer, which is by definition, the Euler number of E relative to τ . Now, given the oriented Seifert fibration π : M → S, let us remove from S small, pairwise disjoint, open discs around the points corresponding to the special fibres, and denote by B what is left. Let E be π −1 (B), which is M minus a union of open solid tori. This is an S1 bundle over B. On each boundary torus Ti one can can choose a unique (up to isotopy) oriented curve a which intersects each Seifert fibre in exactly one point and satisfies that m = α[a] + β[b], where m is a meridian of Ti , (α, β) are the corresponding reduced orbit invariants and [b] is the homology class represented by one Seifert fibre. This curve a determines a section of E|Ti . Doing this for each boundary torus we obtain a section of E over ∂B. The Euler number e = e(M ) of the Seifert fibration π : M → S is defined to be the Euler number of E relative to the given trivialization over ∂B. 1.2 Definition. The normalized (or reduced) Seifert invariants of the Seifert manifold M are: { g; e; (α1 , β1 ), . . . , (αr , βr ) }. Now, given the surface V with C∗ -action, let V ∗ = V − {0}. Consider the subgroup R+ ⊂ C∗ of positive real numbers. If we divide V ∗ by the induced R+ action we obtain the link M , which is then naturally equipped with an S1 -action. Thus M becomes a Seifert manifold. The orbit space S = V ∗ /C∗ is an oriented 2-dimensional closed manifold, π canonically diffeomorphic to the orbit space M/S1 . The projection V ∗ −→ S ∗ endows V with the structure of a holomorphic Seifert bundle. This means that V ∗ minus the exceptional orbits is indeed a holomorphic C∗ -bundle over S minus the corresponding points, and the structure of V ∗ around the exceptional orbits is described by the orbit invariants (αi , βi ), which can be defined as follows. Let Oi be the corresponding exceptional S1 -orbit on M . The isotropy group of Oi is necessarily a finite cyclic subgroup σi of S1 , and its order is the corresponding invariant αi . Now, given a point x in Oi , let H be a 2-plane in the tangent space Tx M transversal to the line tangent to the orbit. Using a local chart, we identify

III.1. Quasi-Homogeneous surface singularities

69

ˆ ⊂ H with a small disc U in M , which is transversal to Oi . Now let a small disc U W be the union of all the S1 -orbits that meet U . This is a solid torus, and it is a neighborhood of Oi in M . By the slice theorem (see for instance [193, 256]), the S1 action gives a representation σ ˆ of the isotropy group σi in GL(2, R), regarded as the ˆ) = U ˆ for all γ ∈ σ, and W is equivariantly automorphisms of H, such that σ ˆ (γ)(U  1 ˆ ˆ by γ ·(g, u) = (gγ −1 , σi (γ) u). diffeomorphic to (S × U ) σi , where σi acts on S1 × U We may identify the plane H with C and the group σi with the αth roots of unity, so that the slice representation is just complex multiplication by these roots of ˆ the slice representation takes the unity. Then, for each γ in σi and for each x ∈ U νi form σ ˆ (γ)(z) = γ (z), for some integer νi , 0 < νi < αi . The integers (αi , νi ) are the orbit invariants of Oi as used in [194, 256]. The corresponding invariant βi is defined by νi βi ≡ −1 mod (αi ) ; so it is the inverse of −νi modulo αi . As an example (see [256]), consider the affine surface V defined by z112 + z28 − 5 z2 z3 = 0. Then V is invariant under the action t · (z1 , z2 , z3 ) → (t10 z1 , t15 z2 , t21 z3 ), so V is quasi-homogeneous and it has six exceptional orbits. One of these orbits is O1 = {(z1 , z2 , z3 ) ∈ V ∩ S5 | z1 = z27 − z35 = 0 } , whose isotropy group is Z3 , so α1 = 1. To compute β1 we notice that (0, 1, 1) is a point in O1 and a plane transversal to this orbit is given by z2 = z3 = 0. Then the slice representation is γ · (z1 , 0, 0) = (γ 10 z1 , 0, 0) for γ a cubic root of unity. Thus ν1 ≡ 10 ≡ 1 mod 3. Therefore β1 = 2. The Euler number e also has a very nice interpretation, coming from [204, 187]. For this, as in Chapter II, we let P denote one of the standard simply connected complex manifolds of dimension 1: CP1 , C or the upper half-plane H, which serve as models for the classical plane geometries. Since the bundle V ∗ over S is holomorphic, it turns out that the surface S is automatically a Riemann surface with marked points corresponding to the exceptional orbits, where it has an orbifold structure. By the Riemann-K¨ obe uniformization theorem we know that choosing the appropriate “plane”, one has that P is the universal cover of S (as an orbifold) and there exists Γ ⊂ Aut(P ), a discrete group of holomorphic automorphisms of P with quotient S = P/Γ. Then a well-known theorem of  of Γ of finite Selberg (see also [80]) says that there exists a normal subgroup Γ H index which acts freely on P . Thus one has a finite covering map S → S with  It was noted in [204] (see also [171, 6.6]) that there is a covering group H = Γ/Γ. ∗ holomorphic C -bundle L over S with an action of H (something like a “pull-back bundle”) with L/H ∼ = V ∗ , and one has a commutative diagram: H

L −−−−→ ⏐ ⏐ π

V∗ ⏐ ⏐ π

H S −−−−→ S .

(1.3)

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We recall that the C∗ -bundle L over S is classified topologically by its Chern class  Z), which evaluated on the fundamental cycle [S]  gives the Euler c1 (L) ∈ H 2 (S; number of L defined as above (see also Chapter IV). It turns out that the number e(V ∗ → S) = (Euler number of L)/order of H ∈ Q

(1.4)

 and the bundle L. This is independent of the choice of the normal subgroup Γ number is called the virtual degree, or rational Euler number of the Seifert bundle V ∗ → S, and it is related to the Euler number e in 1.10, which is an integer, by the formula: m  βi . e(V ∗ → S) = −e − α i=1 i The rational Euler number e(V ∗ → S) of the Seifert bundle generalizes the usual Euler number of S1 -bundles; this was introduced in [187, 204] for Seifert manifolds M in general, and it was shown that the corresponding Seifert manifold occurs as the link of a surface singularity if and only if e(M → S) < 0. In [194], completing previous results by several authors, there is given an explicit method for computing the Seifert invariants of all quasi-homogeneous surface singularities from the the weights of the corresponding C∗ -action, and they use this information to determine the weighted graph of a canonical good resolution for these singularities (c.f. Chapter IV below). 1.5 Remark. It is well known that there is a very close relation between quasihomogeneous singularities, graded algebras of finite type and automorphy factors. The theorems of Klein, Milnor and Neumann in Chapter II are an example of this, and we refer to [256] for more on the subject. Given a quasihomogeneous surface V as above, an automorphy factor for V means a diagram similar to (1.3) but replacing S by the universal cover P of S. More precisely, given the C∗ -bundle π V ∗ → S, then (by [204, 54, 59]) there exists a holomorphic line bundle L over P with an action of Γ that commutes with the C∗ -action, such that L/Γ = V ∗ and one has a commutative diagram: Γ

L −−−−→ ⏐ ⏐ π

V∗ ⏐ ⏐ π

(1.6)

Γ

P −−−−→ S . ∼ S2 , and The triple (P, L, Γ) is called an automorphy factor for V . If P is CP1 = 1 1 ∗ Γ ⊂ SU (2), then the canonical automorphy factor is (CP , T (CP ), Γ), where T ∗ (CP1 ) is the cotangent bundle. When P is the upper half-plane H, and Γ ⊂ P SL(2, R), the canonical automorphy factor is (H, H × C, Γ), since the tangent bundle of H is trivial, T (H) ∼ = H×C; the action of Γ on T (H) is via the derivative, as explained in II.5 above. 1.7 Remark. It is clear that if V is a quasi-homogeneous singular variety, then the C∗ -action on V defines a holomorphic vector field on V , whose orbits are

III.2. 3-manifolds covered by Lie groups

71

everywhere transversal to the link. This vector field plays the role of a “radial” vector field. One can prove (see [29]) that every small perturbation of this vector field is still “equivalent” to it in some sense that can be made precise, and these types of vector fields form an open dense set in the space of all germs at 0 of holomorphic vector fields on V . This motivates two questions: Question 1: If an isolated complex singularity (V, 0) admits a holomorphic vector field which is singular only at 0 and the field of complex lines that it spans is everywhere transversal to the link, does it follow that V is quasi-homogeneous? If this is true, it is probably a consequence of the deep results of K. Saito in [216], where he gives several characterizations of quasi-homogeneous singularities. Question 2: Given an isolated singularity germ (V, 0), one can always make sense of the concept of “the local radial (or Schwartz) index” of germs of vector fields on V (see for instance [3, 35, 69]); the results of [29] imply that there is a smallest possible index attained by the holomorphic vector fields on V . If V is quasihomogeneous, then this index is 1. What is this number for isolated singularity germs which are not quasi-homogeneous? An alternative way of re-phrasing this question is to characterize the generic germs of holomorphic vector fields on V with the “least complicated topology”.

III.2

3-manifolds whose universal covering is a Lie group

In this section we describe the closed, oriented 3-dimensional manifolds which are diffeomorphic to homogeneous spaces of the form Γ\G, where G is a 3-dimensional Lie group and Γ is a discrete subgroup of G with compact quotient; for short we call these uniform subgroups, as it is common in the literature. Our basic reference is the article of Raymond and Vasquez [207], which completes previous work by various people. In fact the problem of classifying the closed 3-manifolds of the form Γ\G was considered in [23, Ch. III], where the authors give the list of the 3-dimensional Lie groups that admit uniform subgroups; they also list the corresponding uniform subgroups, but their list is incomplete and erroneous in some cases. There is also a classification of the 3-dimensional Lie groups with uniform subgroups given in [172, §4 ]. It turns out that in all cases but one, the corresponding orbit spaces Γ\G admit circle actions and therefore are Seifert manifolds. Thus the classification in [207] is based on the classification given in [206] of circle actions on 3-manifolds. The remaining case is when G is the solvable group E(1, 1) which had to be treated separately. Notice that it is enough to consider the simply-connected Lie groups, for given another one, we can always lift it to the universal covering and work there. Up to isomorphism, there are six different 3-dimensional, simply-connected Lie groups that admit uniform subgroups. These are: (i) two semi-simple groups:
SU (2) (compact) and SL(2, R), both of them known to us from Chapter II; two

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 + (2), which is the universal covering of the group solvable (not-nilpotent) groups: E + E (2) of orientation preserving isometries of the euclidean plane, and the inhomogeneous Lorenz group E(1, 1); the Heisenberg group N of 3 × 3 upper triangular real matrices, which is nilpotent, and R3 regarded as an Abelian (additive) group. We recall briefly which are the uniform subgroups in each case, and which are the corresponding quotient manifolds. Except for the case G = SU (2), in all other five cases the underlying manifold is R3 , with different Lie group structures. It is worth noting that these Lie groups give rise to six of the eight geometries which are relevant for 3-manifolds theory according to W. Thurston (see [186]). Let us describe the six types of manifolds of the form Γ\G. (i) If G = SU (2), this was widely studied in the previous chapter. Its finite subgroups are either cyclic or the triangle groups Γ(2,2,r) , r ≥ 2, Γ(2,3,3) , Γ(2,3,4) , Γ(2,3,5) . The quotient Γ\SU (2) is a Seifert manifold which fibres over (Γ(p,q,r) \S2 ) ∼ = S2 with either two or three exceptional fibres, according as the group Γ is cyclic or a triangular group. In the second case the special orbits are those of the vertices of the triangle and the corresponding Seifert invariants are: {0 ; −2 ; (p, p − 1), (q, q − 1), (r, r − 1) }. When Γ = Zn is cyclic, the manifold Γ\SU (2) is the lens space L(n, 1), which is also a Seifert manifold but it has many such representations, unlike all the other cases.
(ii) Let G = SL(2, R). We start with P SL(2, R); its discrete subgroups are called Fuchsian groups, and they are said to be cocompact when the quotient Γ\G is compact, which is the relevant case for us. Examples of these are the triangle and the polygonal groups of Chapter II. The elements of P SL(2, R) are of three types: hyperbolic, parabolic and elliptic; the elliptic elements are conjugate to rotations and are the only type of elements in P SL(2, R) that have fixed points is H: the hyperbolic elements have two fixed points on the real axis (which is not in H), and the parabolic elements have one fixed point on the real axis. A Fuchsian group Γ can have at most finitely many orbits of fixed points in H, and the isotropy of each orbit is cyclic of finite order. Let us denote by α1 , . . . , αm the orders of the isotropy subgroup of the different orbits of fixed points. For instance, for the triangle groups these are precisely p, q, r. The group Γ acts on H and the quotient is a Riemann surface of genus g ≥ 0, which has an orbifold structure with marked points x1 , . . . , xm , which correspond to the orbits with non-trivial isotropy. Thus one has a branched covering projection H → Γ\H, ramified at these m points, with branching indices the αi . The set of integers {g; α1 , . . . , αm } is called the signature of the Fuchsian group, and it characterizes the group up to conjugation by a quasi-conformal diffeomorphism (see for instance [28]). For instance, if Γ is the fundamental group of a Riemann surface S of genus g > 1, then Γ is a subgroup of P SL(2, R) that acts freely on H with quotient S; hence the signature is in this case {g; 0}. The projection p : H → Γ\H = S is now

III.2. 3-manifolds covered by Lie groups

73

a covering projection, without ramifications, and the orbit space Γ\G is the unit tangent bundle of S. For a triangle group the signature is {0; p, q, r}. In general, there is a projection Γ\G → Γ\H which is a Seifert fibration, with exceptional fibres over the m points x1 , . . . , xm which have non-trivial isotropy. The orders αi of these isotropy groups gives the orbit invariants αi of Definition 1.2.
Now recall that G = SL(2, R) is a central extension of P SL(2, R) and one has an exact sequence: ∞
P SL(2, R) → 1 . 0 → Z → SL(2, R) −→

π

For every positive integer r ≥ 1 one has an r-fold cyclic cover Gr of P SL(2, R) and projections πr
R) −→ Gr −→ G1 = P SL(2, R) −→ 0 . G∞ = SL(2,

In fact one has central extensions, π

r G1 −→ 1 , 0 −→ Z/r −→ Gr −→

pr

0 −→ Z −→ G∞ −→ Gr −→ 1 . According to [207], each discrete cocompact subgroup Γ∞ ⊂ G∞ has finite index, −1  (π∞ (Γ∞ )). For instance, if Γ ⊂ P SL(2, R), then its lifting Γ say r = r(Γ), in π∞ −1
to SL(2, R) obviously has index r = 1 in π∞ (π∞ (Γ∞ )); if we now start with a  to SL(2,
subgroup Γ ⊂ SL(2, R), then its lifting Γ R) may have r = 1 or r = 2 according as Γ contains or not the kernel of the projection SL(2, R) → P SL(2, R), respectively. One has that the projection Γr of Γ∞ to the r-fold covering group Gr only meets the kernel of the projection pr : Gr → G1 at the identity and one has Γr \Gr ∼ = Γ∞ \G∞ .
Then, according to [207], the cocompact discrete subgroups of SL(2, R) and the diffeomorphism type of the quotient MΓ = Γ∞ \G∞ , are characterized by the corresponding integer r, together with the signature of its image Γ1 = π∞ (Γ∞ ) in P SL(2, R). As mentioned before, these quotients are all Seifert manifolds; their (normalized) Seifert invariants can be computed from the signature of Γ1 . These are (see [207]): { g; e = 2g − 2; (α1 , α1 − 1), . . . , (αm , αm − 1) } , where g is the genus of Γ1 \H = S. The corresponding integer r was found in [231, 2.4] to be: χ(MΓ → S) , r = e(MΓ → S) m βi where e(MΓ → S) = i=1 αi is defined in (1.4) above, and the Euler characteristic χ(MΓ → S) of the Seifert fibration is defined by: χ(MΓ → S) = χ(S) −

m  αi − 1 i=1

αi

.

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The invariant χ(MΓ → S) appears in Pinkham’s and Dolgachev’s work. It is shown in [186, p. 250–251] that it distinguishes the possible geometries in the link
M = Γ\G of a quasi-homogeneous singularity: the group G is SU (2), SL(2, R) or the Heisenberg group N according as χ(MΓ → S) is > 0, < 0, or = 0, respectively (see the discussion below). (iii) G = N , the Heisenberg group of real ⎛ 1 x ⎝0 1 0 0

matrices of the form ⎞ t y⎠ . 1

It has one discrete subgroup Γk for each integer k ≥ 1, which consists of the elements in N whose entries are integral multiples of k. The manifolds one gets are S1 -bundles over the torus T = S1 × S1 . Notice one has a natural projection of N into R2 regarded as the plane defined by the x, y coordinates, whose quotient by the action of Γk is the 2-torus. The fibre of this projection is R, spanned by the t-coordinate, which maps into the S1 -fibres of the S1 -bundle over the torus. As noticed in [171, p. 222], the first integral homology group of the quotient Γk \N is isomorphic to Z ⊕ Z ⊕ Zk , so the Euler number of this bundle is ±k, the sign depending on the choice of orientations. This happens because the group operation on N is given by: [x, y, t] · [x , y  , t ] = [x + x , y + y  , xy  + t + t ] , where we are writing [x, y, t] for the corresponding matrix (c.f. the Abelian case G = R3 below). iv) G = E(1, 1). This is the inhomogeneous Lorentz group of affine transformations of R2 that preserve the quadratic form x2 − y 2 . This group and its discrete subgroups are beautifully explained in [104]. One has an extension 1 → R2 → G → R → 1 with [G, G] = R2 , which presents G as a semi-direct product of R2 and R. Its discrete subgroups are all of the form Γ = (Z × Z)
Z := M
V. The quotient of R2 by the lattice M is a 2-torus T2 , while the quotient of R by V is a circle. The quotient Γ\G is a torus bundle over S1 with infinite cyclic monodromy, where both eigenvalues are positive and distinct from 1; all such possibilities occur.  + (2), the universal cover of the affine group E + (2), which can be regarded v) G = E as the group of real matrices: ⎞ ⎛ cos 2πθ − sin 2πθ x1 ⎝ sin 2πθ cos 2πθ x2 ⎠ . 0 0 1

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75

The group G is a semi-direct product R2  R, where R acts on R2 by the rotations induced from SO(2) ⊂ E + (2). It is a central extension of E + (2),  + (2) → E + (2) → 1 . 0→Z→E We see from [207, 1.3] that every uniform subgroup of G is the lifting of one such subgroup in E + (2), and these are Z2 = Z ⊕ Z, the triangle groups (2, 3, 6), (2, 4, 4), (3, 3, 3) and the quadrangle group (2, 2, 2, 2). For Γ = Z2 the quotient MΓ = Γ\G is the 3-torus T3 , but regarded as a homogeneous space of E + (2), not equivalent to its structure as an Abelian Lie group considered below. In the other cases the corresponding quotient MΓ is a Seifert manifold with Seifert invariants { 0; −2; (α1 , α1 − 1), . . . , (αn , αn − 1) }, where n is either 3 or 4 and (α1 , . . . , αn ) is one of the above triples (if n = 3) or quadruple (if n = 4). (vi) G = R3 . In this case the only discrete subgroup with compact quotient is Z3 = Z ⊕ Z ⊕ Z, up to equivalence, and the quotient is the 3-torus T3 regarded as an Abelian group with the product structure. This classification of 3-manifolds which are quotients of 3-dimensional Lie groups by discrete subgroups motivates the following interesting question, asked of me some time ago by Etienne Ghys, whose answer I do not know: 2.1 Question. Which 3-manifolds arise as quotients H\G, where G is a Lie group of dimension n + 3 and H is a subgroup of G of dimension n? That is, which 3-manifolds are homogeneous spaces? Recently E. Ghys mentioned to me that he thinks this question has been answered already, but I could not find it in the literature. Of course, once we know the answer then a natural next problem is to decide which of these homogeneous 3-manifolds are links of complex surface singularities? More generally one may study which closed 3-manifolds arise as orbit spaces of the form M = ρ(H)\X, where X is a smooth Riemannian manifold, H is a connected Lie group and ρ is an action of H on X whose orbits are all of codimension 3 with the same isotropy everywhere. This is closely related to the problem of classifying cohomogeneity 3 actions, which is no doubt an interesting and hard problem (c.f. [174, 106, 98, 245]).

III.3

Lie groups and singularities I: quasi-homogeneous singularities

One has the following theorem of [59, 186], which culminates from the theorems of Klein, Milnor and Neumann described in Chapter II, as well as previous results by Dolgachev in [57, 58], see also [187]. 3.1 Theorem. Let (V, 0) be the germ of a quasi-homogeneous surface singularity which is Gorenstein. Then its link M is diffeomorphic to an orbit space of the

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Chapter III. Lie Groups and Singularities


form Γ\G, where G is one of the groups SU (2), SL(2, R) or N , and Γ is a discrete subgroup of G. Conversely, let G be one of these three Lie groups and Γ ⊂ G a discrete subgroup with compact quotient MΓ = Γ\G; then MΓ is diffeomorphic to the link of a normal, Gorenstein quasi-homogeneous surface singularity. We recall that an isolated surface singularity germ (V, 0) is (analytically) normal if every bounded holomorphic function on V ∗ = V − 0 extends to a holomorphic function at 0 (see for instance [179]). This implies that the singularity is isolated. (For hypersurface or complete intersection germs these two conditions are actually equivalent, i.e., normal iff the singular set has codimension more than 1.) A normal surface singularity germ (V, 0) is Gorenstein if there exists a nowherevanishing holomorphic 2-form on V ∗ . [Notice that the canonical bundle K of V ∗ , i.e., the bundle of holomorphic 2-forms, is 1-dimensional. Hence the existence of a nowhere-vanishing holomorphic 2-form on V ∗ is equivalent to saying that K is holomorphically trivial. By normality, this means that the dualizing sheaf of V is free at 0, which is the usual requirement for a singularity to be Gorenstein.] It is worth saying that one cannot expect to have in this generality such an explicit description of the equivalence between the orbit spaces Γ\G and the corresponding singularities, as one does in Milnor’s theorem for the Brieskorn manifolds (or in its generalization by Neumann described in Chapter II), because in the general case the structure of the local ring of these singularities can be quite complicated, with many generators and relations. However one does have that in all cases the corresponding local rings are given by appropriate automorphic forms (see [59]). Moreover (see [183, 186]), their analytic structure may have a large-dimensional deformation space, so one has to look also at the embedding of the group Γ in the corresponding group G, similarly to classical Teichm¨ uller theory. This means in particular that the singularity one gets from a given discrete subgroup Γ ⊂ G does not depend only on the abstract group Γ, but also on the way it is embedded in G. We refer to the articles of Neumann and/or Dolgachev for the proof of this theorem and for the interesting study in [186] of the deformations of both, the analytic structure on the variety and the embedding of the group Γ in G. Here we follow [231] and describe how to construct explicitly the surface singularities associated to the orbit spaces Γ\G in each of the three cases, and we show that these singularities are Gorenstein by exhibiting explicit nowhere-vanishing holomorphic 2-forms in all cases. This construction is essentially Dolgachev’s and the arguments below rely on his work, specially in [59]. 3.2. In the case G = SU (2) the corresponding singularities are those of Klein studied in Chapter II. It is clear that these singularities admit a good C∗ -action, and they are normal (since they are isolated hypersurface singularities, or also by Grauert’s contraction criterium that we explain in Chapter IV). The corresponding 2-form on VΓ = Γ\C2 is obtained by noticing that the holomorphic form Ω = dz1 ∧ dz2 on C2 is clearly nowhere-vanishing and invariant under the action of

III.3. Lie groups and quasi-homogeneous singularities

77

all SU (2), which acts on C2 by holomorphic transformations. Hence this form descends to the quotient VΓ and is nowhere-vanishing on VΓ∗ = VΓ − 0. Alternatively, one may consider (say for the commutators of the binary triangle groups) the corresponding Brieskorn variety in C3 , Vp,q,r = { z1p + z2q + z3r = 0 } , whose link is G = SU (2) divided by the commutator of the corresponding triangle group (one has analogous singularities for all the finite subgroups of SU (2), see  = dz1 ∧ dz2 ∧ dz3 ; this induces a canonical Chapter II). One has in C3 the form Ω  with the gradient vector field ∇f , 2-form ω on Vp,q,r −0, obtained by contracting Ω where f is the corresponding Pham-Brieskorn polynomial. In local coordinates ω is: ω=

dz1 ∧ dz2 dz2 ∧ dz3 dz3 ∧ dz1 = = . ∂f /∂z3 ∂f /∂z1 ∂f /∂z2

(3.2.1)

It is easy to see that the 2-forms on the right-hand side of this equation, which are defined where the denominator is not zero, coincide when any two of them are defined and so they give a global 2-form, which coincides with the one given by  with ∇f . It is now an exercise (see [229]) to show that the pull-back contracting Ω of this 2-form w to C2 under Klein’s map C2 → C3 is a constant multiple of the form Ω = dz1 ∧ dz2 , and therefore these two forms on VΓ = Γ\C2 coincide up to a constant.
3.3. Let us describe now the case G = SL(2, R). We consider first the case Γ ⊂ P SL(2, R). The corresponding singularities are known as Dolgachev singularities and were introduced in [57, 58]. We recall (Chapter II) that P SL(2, R) acts on the tangent bundle T H of the hyperbolic plane by: 
 dg g · (z, w) → g(z) , w . dz This action is free away from the zero section, which is H and where we may have fixed points, all with finite cyclic isotropy. Thus we may embed P SL(2, R) in T H as the boundary of a tubular neighborhood of H in T H, regarded as a complex manifold. The orbit space VΓ = Γ\T H is a complex analytic surface with (possibly) singularities contained in the Riemann surface Γ\H ⊂ VΓ , which correspond to the special orbits of the Γ-action on H. Then Dolgachev showed that Γ\H can be blown down to a point that we may denote by 0 (this follows essentially from Grauert’s criterium, c.f. Chapter IV). The result is a complex analytic surface VΓ with an isolated normal singularity at 0. It is clear from the construction that the link of VΓ is Γ\P SL(2, R). One may exhibit an explicit 2form for these singularities, thus proving that they are Gorenstein, but we will do
this in general below for all the uniform subgroups of SL(2, R). It is worth saying that the singularities one gets in this way may not be hypersurfaces nor complete intersections, and not even smoothable, as shown by

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Chapter III. Lie Groups and Singularities

Pinkham. For instance, for the triangle subgroups of P SL(2, R), Dolgachev showed that there are only 14 of them for which the corresponding singularity can be defined by a single equation; these correspond to the 14 exceptional unimodal singularities of Arnold (c.f. [257]). To construct the singularities corresponding to arbitrary uniform subgroups
of SL(2, R) we adopt the notation introduced in [171, 231]. 3.4 Definition. For every integer r ≥ 1, an r-labeled biholomorphic map of H to itself  df means a biholomorphic map f : H → H, together with a lift dz of its derivative to the r-fold cover of the multiplicative group C∗ . Taking r = 1 we recover the group G1 = P SL(2, R) as the group of 1-labeled biholomorphic maps of H, while for r = ∞ we get the labeled biholomorphic maps
of Milnor, which form the group SL(2, R). It is an exercise to see that the set of all r- labeled biholomorphic maps forms the r-fold cyclic covering group Gr of P SL(2, R). If we denote by Cr ∼ = C the r-fold cyclic cover of C branched at 0, we then have an action of Gr on H × Cr , which embeds Gr as the orbit of (i, 1), where 1 is a selected point in Cr lying over 1 ∈ C. Given Γ ⊂ Gr , we have an analytic surface Γ\H × Cr with isolated singularities on SΓ = Γ\(H × 0); the 3-manifold Γ\Gr = MΓ is embedded in Γ\H × Cr as the boundary of a tubular neighborhood of SΓ . Observe that we are now having an automorphy factor: as in [57, 58, 59], we may now blow down SΓ to a point (essentially by Grauert’s criterium, c.f. Ch. IV) to obtain a complex analytic surface VΓ with a normal singularity at a point that we may denote by 0, with an obvious C∗ -action. The link of VΓ is Γ\Gr = MΓ . To show that this singularity is Gorenstein we observe, following [229], that  ∗ the holomorphic 2-form: one has on H × C Ω =

dz ∧ dw , w2

which is nowhere-vanishing and, we claim, is invariant under the action of

SL(2, R). In fact, for every g ∈ SL(2, R) we have: g ∗ dz ∧ g ∗ dw g Ω(z, w) =  dg 2 = w2 dz (z) ∗

  dg

2 

dz (z)  dg 2 dz (z)

w2

dz ∧ dw =

dz ∧ dw , w2

so Ω is invariant. Hence it descends to a nowhere-vanishing holomorphic form on Γ\H × (Cr − 0), and so defines a nowhere-vanishing holomorphic 2-form on VΓ − 0.
3.5 Remark. We notice that in the two cases above, G = SU (2) or G = SL(2, R), one has free actions of G on a certain complex 2-manifold X, which is C2 − 0  ∗ . In both cases the G-orbits foliate X by copies of G, and one has a or H × C G-invariant nowhere-vanishing holomorphic form on all of X. We will see that one has a similar picture for all the six Lie groups in question.

III.3. Lie groups and quasi-homogeneous singularities

79

3.6. Consider now the Heisenberg group N of real matrices of the form ⎛ ⎞ 1 x t ⎝0 1 y ⎠ . 0 0 1 For simplicity we denote such a matrix by [x, y, t]. The group operation on N is given by: [x, y, t] · [x , y  , t ] = [x + x , y + y  , xy  + t + t ] . ρ

We may define the smooth map N × C2 −→ C2 by: ρ([x, y, t], (z1 , z2 )) = (z1 + x + iy, e2πit z2 ) . This is not a group action since:   
ρ [x, y, t], ρ [x , y  , t ], (z1 , z2 ) = (z1 + x + x + i(b + b ), e2πi(t+t ) z2 ) , while   

ρ [x, y, t]·[x , y  , t ] , (z1 , z2 ) = (z1 +x+x +i(b+b ), e2πi(t+t +xy ) z2 ) , (3.6.1)


so we get an extra factor e2πi(xy ) in the second variable. Still we observe that ρ is an action restricted to every discrete subgroup of N , since in these cases xy  ∈ Z. Notice that we can embed C in N by z → [x, y, 0], though this is not a group homomorphism. Still, the restriction to C ⊂ N of the map ρ is a group action of C on C2 , acting by translations in the first variable. We may also define an embedding of R in N (this time as a subgroup) by t → [0, 0, t]. The map ρ gives an action of R ⊂ N on C2 , it is multiplication in the second variable by e2πit , which are rotations. It follows that for each (z1 , z2 ) ∈ C2 with z2 = 0, the image of the map: ρ(z1 ,z2 ) : N → C2 given by ρ(z1 ,z2 ) ([x, y, t]) is a cylinder C × S1 . This defines a foliation of C2 by cylinders, with one special leaf L0 = {(z1 , 0)} (where the foliation is singular), and every discrete subgroup Γ of N acts on each leaf of this foliation. By the previous comments we see that one also has an S1 -action on each leaf of the foliation, given by the restriction of ρ to R × C2 . We now recall that, up to isomorphism, N has one discrete subgroup Γk for each integer k > 0, consisting of those triangular matrices as above, for which the coefficients x, y, t are all multiples of k. Given Γk = Γ ⊂ N discrete, we can pass to the quotient VΓ = Γ\C2 . The singular leaf L0 becomes a 2-torus T2 = Γ\L0 , and all other leaves of F become copies of MΓ = Γ\N ∼ = Γ\(C × S1 ), so they are 1 S -bundles over the torus and the complex surface VΓ is a holomorphic line bundle over the 2-torus. The Chern class of this bundle is ±k, depending on the choices of orientations, due to the contribution of the {x, y}-variables in (3.6.1). This means

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that MΓ is a circle bundle over the 2-torus with Euler class ±k. Choosing the orientations so that the self-intersection of T2 is −k, we have that the contraction criterium of Grauert says that we can blow T2 down to a point to get a surface singularity VΓ , whose minimal resolution (see Chapter IV) is VΓ . For k = −1, −2, −3 the surfaces one gets in this way are homeomorphic to the Brieskorn varieties, V(p,q,r) = { z1p + z2q + z3r = 0 } , where (p, q, r) is one of the triples (2, 3, 6), (2, 4, 4) or (3, 3, 3). This follows either by [171], by [194] or by [59, 186]. Milnor tells us that the corresponding Brieskorn manifolds are circle bundles over the torus, with Euler class equal to −k, where k is the greatest common divisor of (p, q, r); Orlik and Wagreich [194] tell us that the minimal resolutions of these singularities are holomorphic line bundles over the 2-torus, with the corresponding Chern classes. We remark that Dolgachev in [59] gives a finer argument than we do here, and he proves the above diffeomorphisms directly. We also remark that these singularities are elliptic, meaning by this that their geometric genus is 1 (see [122] and Chapter IV below). Moreover, by [122] they are also minimally elliptic, which amounts to ellipticity plus Gorenstein. To see that they are actually Gorenstein we observe, following [231], that for every h = [x, y, t] ∈ N the map θh : C2 → C2 given by: θh (z1 , z2 ) = ρ([x, y, t], (z1 , z2 )) = (z1 + (x + iy) , e2πit z2 ) is holomorphic and leaves invariant the holomorphic 2-form Ω =

dz1 ∧ dz2 z2

since for every g = [x, y, t] ∈ N one has: g ∗ Ω(z1 , z2 ) =

g ∗ dz1 ∧ g ∗ dz2 dz1 ∧ e2πit dz2 = = Ω(z1 , z2 ) . e2πit z2 e2πit z2

Hence this form descends to VΓ − 0 and is never-vanishing there.

III.4




Lie groups and singularities II: the cusps

This section is essentially taken from [104], where Hirzebruch constructs and studies the cusp singularities. Let K be a totally real quadratic field over Q, so it has two different embeddings in R, that we denote by x(1) , x(2) , for x ∈ K. Let  be the ring of algebraic integers in K, and let x → x be the non-trivial automorphism of K. The Hilbert modular group:  a b  

SL2 () =

a, b, c, d ∈  , ad − bc = 1 c d

III.4. Lie groups and singularities II: the cusps

acts on H × H, where H  a c

81

is the upper half-plane, by   az + b a z + b  b 1 1 . , (z1 , z2 ) = d cz1 + d c z1 + d

The corresponding projective group P = SL2 ()/{±Id} acts effectively on H × H, and for each point x ∈ H × H the isotropy group Px ⊂ P is finite and cyclic. The orbit space P \(H × H) is a complex space, which can be compactified by adding to it a finite number of points, called the cusps. The resulting space is a compact algebraic surface with isolated singularities at these cusps, whose links are torus bundles over the circle, diffeomorphic to quotient spaces of the form Γ\E(1, 1). More generally, given K, let M be a Z-module of rank 2 contained in K, + be the group of totally positive units in K that leave M invariant; we and let UM recall that an element x ∈ K is totally positive (denoted x  0) if each of its two + embeddings in R is > 0. The elements of UM are automatically algebraic integers + + ∼ is free of rank 1, UM since they leave M invariant (see [104]). The group UM = Z. We recall that the group P SL(2, R) acts on H via the M¨ obius transformations; thus its product P SL(2, R)2 = P SL(2, R) × P SL(2, R) acts naturally on H × H. An element of P SL(2, R) is parabolic when it has exactly one fixed point in CP1 , and in this case the fixed point is in RP 1 ∼ = R ∪ ∞. Similarly, an element γ = (γ (1) , γ (2) ) of P SL(2, R)2 is called parabolic iff both γ (1) , γ (2) are parabolic. Such an element has exactly one fixed point in (CP1 )2 , which belongs to (RP 1 )2 . A discrete subgroup Γ ⊂ P SL(2, R)2 is said to be irreducible if it contains no element γ = (γ (1) , γ (2) ) such that γ (i) = 1 for some i and γ (j) = 1 for some j. It is proved in [235] that an irreducible discrete subgroup Γ ⊂ P SL(2, R)2 has at most finitely many distinct orbits of parabolic points. Let us look at the geometry near a parabolic point. According to [235, p. 45], for any parabolic fixed point x of an irreducible discrete subgroup Γ ⊂ P SL(2, R)2 with Γ\H × H of finite volume, the parabolic element ρ ∈ P SL(2, R)2 with ρx = x can be chosen so that ρΓx ρ−1 is contained in P GL(K) ⊂ P SL(2, R)2 , where Γx is the isotropy of x, K is a suitable totally real field and P GL(K) is the projectivization of    a b + GL (2, K) =

a, b, c, d, ∈ K; ad − bc  0 . c d Then one has an exact sequence (see [104, 1.5 (15)]): 0 → M → ρ Γx ρ−1 → V → 1 , where M is a Z-module of rank 2 contained in K and V is a non-trivial subgroup + of finite index. The field K, the equivalence class of M and the group V are of UM completely determined by the parabolic orbit and do not depend on the choice of ρ. In this case one has that ρΓx ρ−1 is a semi-direct product M
V that we may denote by Γ(M, V). The parabolic orbit is called a cusp (similarly to the classical theory of Kleinian groups) of type (M, V), a name which is also used for the image point 0 of this orbit in the quotient space of orbits.

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Chapter III. Lie Groups and Singularities

For any positive number d, the group Γ(M, V) acts freely on W = {(z, w) ∈ H × H | Im(z) · Im(w) ≥ d } . The orbit space Γ(M, V)\W is a non-compact manifold with compact boundary M = Γ(M, V)\∂W . The space Γ(M, V)\W ⊂ Γ(M, V)\H × H can be compactified by adding to it the point 0, whose link is M . By [104, p. 202] the analytic surface VΓ = Γ(M, V)\H × H has a normal singularity at 0. We notice that the group G = E(1, 1), homeomorphic to R3 , can be identified with the group of affine transformations of H × H of the form: (z, w) −→ (t1 z + a1 , t2 w + a2 ) , where a1 , a2 , t1 , t2 ∈ R and t1 t2 = 1. Thus G acts freely on H × H and we can identify G with the orbit of a point, say (i, i). It follows that the link M can be identified with the quotient Γ(M, V)\E(1, 1) and it is therefore a torus bundle over the circle S1 ∼ = V\R. Of course the various orbits of G fill out all of VΓ − 0. We notice too that the holomorphic 2-form Ω =

dz ∧ dw zw

on H×H is obviously invariant under the above action of E(1, 1); it is also invariant under the action of the isotropy group of the cusp, so it descends to a never-zero holomorphic 2-form on a punctured neighborhood of ∞ in VΓ .

III.5

Lie groups and singularities III: the Abelian and E + (2)-cases

It is known ([186, 59]) that the only normal complex surface singularities whose link is of the form Γ\G, with G a Lie group and Γ a uniform subgroup, are those described in Sections 3 and 4 above: the weighted homogeneous Gorenstein singularities and the cusp singularities. It is well known (see for instance [184] or [246]) that the 3-torus cannot be the link of a normal singularity in a complex surface; this observation extends to all quotients of E + (2) by uniform subgroups. Even so, there are interesting similarities among both of these cases and the previous ones, and this is what we discuss in this section, following [231]. We start with the Abelian case, which is somewhat similar to that of the nilpotent case N . Let us recall from Chapter II that one has a canonical isomorphism e : C →  ∗ of the additive (complex) Lie group C with the universal covering group C  ∗ of C  ∗ the the non-zero complex numbers; this isomorphism is obtained by lifting to C 2πiz ∗ 3 : C → C . Given G = R , regarded as a Lie group exponential map e(z) = e

III.5. Lie groups and singularities III

83

∗ under the coordinate-wise addition, one has an action of G ∼ = C × R on C × C given by: ((x, y), t), (z, w)) → (z + (x + iy) , e(t) · w) .  ∗ by copies of R3 embedded as the orbits of the action. The This foliates C × C natural projection into C × C∗ induces an action on C × C∗ given by ((x, y), t), (z, w)) → (z + (x + iy) , e(t) · w) , which extends in the obvious way to an action on C2 whose orbits are all cylinders C × S1 that wrap around the “special leaf” C × {0}. This foliation F is invariant under the action of the whole group G, so in particular it is invariant under Γ = Z3 = Z ⊕ Z ⊕ Z, which is the only uniform subgroup of G. Notice that the action of Z on the second component is already trivial since this is the kernel  ∗ → C∗ . The quotient VΓ = Γ\C2 is a complex manifold which of the map C fibres as a trivial holomorphic line bundle over the torus T2 = Z2 \(C × {0}). The complement of the torus in VΓ is diffeomorphic to a cylinder T3 × R, where T3 = S1 × S1 × S1 ; these 3-torii are quotients of the leaves of the foliation F by the action of Γ. Just as in the nilpotent case, one has the holomorphic 2-form, Ω =

dz ∧ dw w

 ∗ , which is never-vanishing and G-invariant, so it descends to VΓ − T2 . on C × C Hence, so far this case is entirely analogous to the previous cases; the problem now is that T2 is embedded in VΓ with self-intersection 0. Therefore Grauert’s criterium does not apply (see IV.5.8 and IV.5.9) and the torus cannot be blown down complex analytically. 5.1 Question. Can the torus T2 be blown down real analytically? By [4] we know that 5.1 is true up to homeomorphism. Thus, the 3-torus is the link of an isolated singularity in a real analytic 4-dimensional variety (V, 0) which has a holomorphic structure (compatible with the real analytic structure at 0) and has a nowhere-vanishing holomorphic 2-form on V − 0. This seems to be related with an interesting class of singularities that come from the example below, taken from [50], which is the starting point for Chapter VI below. Consider the linear vector field in C3 given by F (z) = (λ1 z1 , λ2 z2 , λ2 z2 ), where the λj = µj + iνj are non-zero complex numbers. Let VF be the variety VF = { z ∈ C3 | F (z), z =

3 

λj |zj |2 = 0 }

j=1

of points where the vector field F is tangent to the sphere through the point and centre at 0. This variety is real analytic since it is defined by the real analytic

84

Chapter III. Lie Groups and Singularities

equations: 3  j=1

µj (x2j

+

yj2 )

= 0 =

3 

νj (x2j + yj2 )

j=1

where zj = xj + iyj . It may consist of 0 ∈ C alone, for instance when the µj are all > 0. However, if we assume that the eigenvalues λi are chosen so that 0 ∈ C is in the interior of the convex hull of {λ1 , λ2 , λ3 }, then one has that: 3

(i) VF has real codimension 2 in C3 ; and 0 ∈ C3 is its only singular point; (ii) the link M of VF is the 3-torus T3 and there is a transitive action of T3 on M ; and (iii) at each point z ∈ (VF − 0) the complex line spanned by F is transversal to VF . The first two conditions tell us that the 3-torus is the link of the real analytic variety VF , which is actually a complete intersection in C3 , and the third condition yields that VF − 0 is, canonically, a complex manifold (see Chapter VI), though it is not (in general) embedded as a complex submanifold of C3 .  + (2), or rather we look at G = E + (2), since Let us consider now the case of E all the uniform subgroups of the former come from the latter. This group acts on C∼ = R2 in the obvious way, and this action extends naturally to the tangent bundle T C = C × C via the derivative:   dg (z) · w . g · (z, w) −→ g(z), dz This action is free away from the line z = 0 and therefore the orbits foliate C × C  + (2) that degenerate into the singular leaf of the foliation, by copies of G = E given by z = 0 . Hence, every discrete subgroup Γ of G = E + (2) acts freely on T C, away from the zero section, but it has fixed points there (except for Γ = Z2 which acts freely also on this line). The quotient VΓ = Γ\T C is a complex analytic surface with zero, three, or four singular points, as the case may be, all contained in the divisor SΓ = Γ\C, which is either a torus, if Γ = Z2 , or CP 1 with three or four marked points in the other cases. The action of Γ on T C preserves the leaves of the above foliation and descends to a foliation of VΓ − SΓ by copies of Γ\E + (2) embedded as boundaries of regular neighborhoods of SΓ , which is the special leaf. Again, as in the Abelian case, one cannot blow down this divisor complex analytically. In the case Γ = Z2 this follows because the divisor SΓ , which is a 2-torus, is embedded with trivial normal bundle. In the other cases this can be seen as follows: at each singularity of VΓ one has, locally, a quotient singularity of the form Zn \C2 for an appropriate n = 3, 4. One can resolve this and get a divisor D in the resolution VΓ ; D consists of a central curve isomorphic to SΓ and one “branch” of rational curves for each singularity of VΓ . Then SΓ can be blown down iff D can be blown down, but an easy calculation shows that the intersection

III.6. A uniform picture of 3-dimensional Lie groups

85

matrix of D is not negative-definite, so Grauert’s criterium says we cannot blow it down. Just as in the Abelian case, one may ask whether SΓ can be blown down real analytically. By [4] this is always the case up to homeomorphism. Finally we observe that the holomorphic form Ω = dz ∧dw on T C is globally defined and G-invariant, so it descends to VΓ minus the singular points, and it is never-zero.

III.6

A uniform picture of 3-dimensional Lie groups

Let us put together some of what we have said in the previous sections about 3-dimensional Lie groups and their uniform subgroups, in relation with surface singularities. Let G be any one of the six 3-dimensional Lie groups with a uniform subgroup Γ ⊂ G. Then we know from the previous discussion that there is associated to G a complex 2-dimensional manifold, equipped with a canonical holomorphic 2-form and a foliation F of X defined, in all cases but one, by an action of G. There are several interesting properties common to all cases, and others that miss in one or two cases. It is convenient to recall first which are the specific manifolds, the 2-forms and the action of G on X in each case. (i) For G = SU (2), X = (C2 − 0) and the form is Ω = dz1 ∧ dz2 . The action is the usual one by linear transformations on C2 .  ∗ ) and Ω = dz∧dw
(ii) For G = SL(2, R), X = (H × C w 2 . The action on H is via the M¨obius transformations, and on C∗ is by lifting the derivative to the universal cover of the multiplicative group C∗ . 2 (iii) For G = N , the Heisenberg group, X = (C × C∗ ) and Ω = dz1z∧dz . This 2 is the only exception in which we do not have a group action, but a map G×X →X ([x, y, t], (z1 , z2 ) → (z1 + (x + iy) e2πit z2 )

which is an action restricted to the uniform subgroups of G. I believe that one should be able to read from [59] all the information we need to fully unify this case with the others, but I have not been able to this this yet. (iv) For G = E(1, 1), X = (H × H) and Ω = dz∧dw z w . The action of G on X is given by identifying this group with the group of affine transformations of X of the form: (z, w) −→ (t1 z + a1 , t2 w + a2 ) , where a1 , a2 , t1 , t2 ∈ R and t1 t2 = 1. (v) For G = E + (2), X = (C × C∗ ) and Ω = dz ∧ dw. Here X is regarded as the non-zero tangent vectors on C. The action on C is the obvious one; on C∗ it is multiplication by the derivative.

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2 (vi) For G = R3 , X = (C × C∗ ) and Ω = dz1z∧dz . The action is given by the 2 same map as in (iii) above, which is now an action.

Having this in mind, let us recall some of the properties that these cases have in common. (a) Except for the case G = N , the leaves of F are the orbits of the G-action, which is free in all cases. (b) In all cases, every uniform discrete subgroup Γ of G acts on X preserving the leaves of F ; and the space of orbits Γ\L is the homogeneous space Γ\G. (c) In all cases one has a canonical holomorphic trivialization of the canonical bundle K of X given by the 2-form Ω, which is G-invariant in all cases but one: G = N , where we have Γ-invariance for all uniform subgroups. This induces an Sp(1) structure on the tangent bundle T X. (d) The orbit space VΓ∗ = Γ\X is a complex analytic manifold, foliated by copies of Γ ⊂ G, with a nowhere-vanishing holomorphic 2-form. For G =
SU (2), SL(2, R), E(1, 1) or N , one has that the complex manifold VΓ∗ is biholomorphic to V − 0, where V is a 2-dimensional surface with a normal singularity at 0. For G = E + (2) and G = R3 ,the complex manifold VΓ∗ is homeomorphic to V − 0, where V is a real algebraic variety with an isolated singularity at 0 (by [4, 4.1]). I believe that the 1-point compactification of VΓ∗ should be itself real analytic, but I do not know how to prove this (c.f. [161, Lemme 1]). Summarizing, one has: 6.1 Theorem. There are, up to isomorphism, six 3-dimensional, simply connected Lie groups with uniform subgroups Γ ⊂ G. Given any such group G and a uniform subgroup Γ, there is associated to G a canonical complex 2-dimensional manifold X, equipped with a canonical holomorphic 2-form and a foliation F of X defined, in all cases but one, by an action of G. The manifold X is in all cases given by an automorphy factor of some line bundle. The quotient VΓ∗ = Γ\X is a complex manifold, foliated by copies of Γ\G, with a canonical never-vanishing holomorphic 2-form. This manifold VΓ∗ is actually an open cylinder Γ\G × (0, 1), and one of its ends can be compactified by attaching to it a smooth divisor SΓ , so that we get a complex analytic surface VΓ = VΓ∗ ∪ SΓ which may have isolated, normal singularities at SΓ . In four of the six cases in question, this divisor can be blown down complex analytically and the result is a complex analytic surface VΓ with a normal singularity P , whose link is the 3-manifold MΓ . In the remaining two cases the divisor SΓ can only be blown down real analytically, so the quotient VΓ is (homeomorphic to) a 4-dimensional real analytic space with an isolated singularity P , whose link is MΓ and which has a complex structure away from P .

III.7. Lie algebras and the Gorenstein property

III.7

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Lie algebras and the Gorenstein property

An essential feature of Lie groups is that they have their Lie algebras, which can be defined as being the vector space of all left invariant vector fields on the group, endowed with the multiplication given by the Lie bracket. These play an important role in the classifications given in [23, 172, 207] of 3-dimensional Lie groups and their uniform subgroups. On the other hand we know from [59, 186] and the previous discussion, that there is a close connection between closed 3manifolds of the form Γ\G and complex surfaces which are normal and Gorenstein, i.e., they admit a nowhere-zero holomorphic 2-form. In this section we discuss an aspect of how the property of being Gorenstein relates to the Lie algebras of the corresponding groups. This goes back to [224, 229, 231]. Given an oriented Lie group G of dimension n, and a positively oriented basis {v1 , . . . , vn } of its tangent space at the identity Te G, multiplication on the left by an element g ∈ G carries Te G isomorphically into Tg G and defines a basis of this space by left translating the basis {v1 , . . . , vn }. In this way we obtain a trivialization L of the tangent bundle T G by left invariant vector fields, and every left invariant vector field is a linear combination of these. Of course this trivialization F of T G depends on the choice of basis for Te G, but this dependence is determined just by a linear transformation at Te G carrying one basis into the other. If Γ is a discrete subgroup of G, then the orbit space MΓ = Γ\G is a smooth manifold, in fact a homogeneous space, and the left-invariant vector fields descend to MΓ . Thus the trivialization L of the tangent bundle T G determines a trivialization of the tangent bundle of MΓ that we also denote L. This trivialization L induces a metric on MΓ with identical curvature properties to the left-invariant metric on G defined by L. This type of metrics on homogeneous spaces plays an important role in geometry (see for instance [172]). One also has several other important geometric structures on MΓ determined by L, as for instance a spin structure with its Dirac operator (see [124] and Chapter IV below). Let us consider now a complex 2-dimensional manifold X. The structure group of its tangent bundle T X is GL(2, C), and we can always reduce it to U (2) by endowing X with some Hermitian metric. If we try to reduce the structure group of T X further to SU (2) we may run into problems, since this is not always possible. The obstruction for doing so is the first Chern class of T X (c.f. IV.1 below). To see this we notice that U (2), being the structure group of T X, acts naturally on the space of differential forms (of all degrees) on X. In particular it acts on the forms of type (2, 0), i.e., on the sections of the bundle associated (2,0) ∗ Tx X; this is isomorphic to the bundle of to T X whose fibre at x ∈ X is holomorphic 2-forms on X, which is called the canonical bundle K of X. The (2,0) ∗ Tx X is by the determinant, and SU (2) is precisely the action of U (2) on subgroup of U (2) which acts trivially on K. Hence we can reduce the structure group of T X from U (2) further to SU (2) if and only if the bundle K is trivial, and the specific reductions of the structure group of T X to SU (2) correspond to the specific trivializations of K. Now, the canonical bundle K is 1-dimensional, so

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it is classified (up to a differentiable isomorphism) by its Chern class, which is the negative of the first Chern class of X (see [101, Th. 4.4.3]. Hence c1 (X) vanishes iff K is trivial and this happens iff we can reduce the structure group of T X to SU (2). Now suppose the manifold X is endowed with a never-zero holomorphic 2form, as in the previous sections. Then this form defines a specific reduction to SU (2) of the structure group of T X. Since SU (2) is isomorphic to the group of unit quaternions Sp(1), this means that we have at each point of X multiplication of tangent vectors by the quaternions i, j, k. Let us now return to the situation envisaged above. In each of the six cases in question we have a specific complex 2-manifold X equipped with a never-zero holomorphic 2-form. Thus we have, by the previous discussion, a specific reduction of the structure group of T X to SU (2) ∼ = Sp(1). Moreover, the normal bundle of the foliation is trivial in all cases. Choosing a trivialization of this bundle one gets, via multiplication by i, j, k, a trivialization L of T F , the bundle tangent to the foliation. If G = N , this trivialization of T F is left invariant under the G-action on X and so defines a basis of the Lie algebra of G. In the nilpotent case we still have the holomorphic 2-form on X and therefore a multiplication by the quaternions i, j, k, which induces a canonical trivialization of the tangent bundle of each leaf of the foliation F . However, there is not a group action of G preserving the leaves, but only an action restricted to each discrete subgroup, so I do not know how to relate these vector fields with the Lie algebra of G. It is worth noting that this same construction applies to the link of every 2-dimensional isolated hypersurface (or complete intersection) singularity [224]. In fact, if V is the germ of an affine surface in C3 defined by a holomorphic mapf germ (C3 , 0) → (C, 0) with an isolated critical point at 0 ∈ C3 , then one has the canonical 2-form ω of 3.2.1 above: ω=

dz2 ∧ dz3 dz3 ∧ dz1 dz1 ∧ dz2 = = . ∂f /∂z3 ∂f /∂z1 ∂f /∂z2

This 2-form defines as above, multiplication by the quaternions i, j, k. The normal bundle of the link M in V ∗ = V − {0} is trivial and canonically trivialized by choosing the unit outwards normal vector field ν (for the restriction to V ∗ of the ambient Hermitian metric). Multiplying ν by i, j, k at each point of T (V ∗ )|M we get a canonical trivialization of the bundle T (M ) tangent to M . This was called in [224] the canonical framing of the link. This also induces a canonical metric and a spin structure on M .

III.8

Remarks

There are several other lines of research, with very important results, that fall into the theme of this chapter, and which I am not including here for several reasons. However I would like to make a few comments in this respect and give

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some guidelines for further reading. Most of what I say here points towards articles of W.D. Neumann, either by himself or with some co-authors. 8.1. It is important, for many reasons, to know when the link of a surface singularity is a homology sphere. It was proved in [185, Th. 1] that every weighted homogeneous surface singularity whose link is a homology sphere, is equivalent to one given by a Brieskorn-Hamm complete intersection (which were described in II.7); and it is relatively easy to decide when the link of a Brieskorn-Hamm complete intersection is a homology sphere. This is explained in Milnor’s book in the case of hypersurfaces in C3 , and for the general case one can use, for instance, the description in [183] of the corresponding Seifert invariants. On the other hand, it is known (see Chapter IV below) that the link of every surface singularity is a plumbed manifold, and there is an operation one can make on plumbed manifolds, called splicing (see [75]), which allows us to construct every link of a singularity which is a homology sphere by iterated splicing of homology spheres which are Seifert manifolds. 8.2. In the previous sections we looked at 3-manifolds of the form Γ\G for G a Lie group and Γ a uniform subgroup, and we discussed the relation of these and surface singularities. There are however much deeper things one can say in this respect. On the one hand, one has the deformations of the complex structure on the corresponding surface singularities; on the other hand, by expressing M as Γ\G we are endowing this manifold with a geometric structure in the sense of Thurston, and this geometric structure can be also deformed. This corresponds to deforming the embedding of the group Γ in G. It is natural to ask which are all these moduli spaces of deformations, and how the deformations of the complex structure of a surface singularity relate to the deformations of the geometric structure on the link. These questions are beautifully answered in [186], where the author also gives references to several significant previous contributions by various authors, including himself. 8.3. Given a complex analytic surface singularity (V, 0) in Cn , we know that the link M of V is the intersection V ∩ Sε of V with a sufficiently small sphere, and we know from Chapter I that the diffeomorphism type of M does not depend on the choice of sphere. In 8.2 above we explained that under certain hypotheses the link M has a geometric structure in the sense of Thurston. Still, it was observed in [220] that one always has another type of geometric structure: since M is a real hypersurface in a complex manifold, it is automatically a CR-manifold. Of course, the CR-structure on M depends on the choice of the sphere, and on the embedding of V in the ambient space. However J. Scherk proved in [220], remarkably, that if (V1 , x1 ), and (V2 , x2 ), are normal surface singularities whose links M1 , M2 are CRisomorphic (for appropriate links), then they are actually analytically equivalent. In fact Scherk proves more: this statement holds in all dimensions, not only in complex dimension 2, and if f is a CR-isomorphism between the links, then f itself extends to an analytic isomorphism between the analytic germs. This was used later in [73] to have a better understanding of the relation between the

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geometric structure of the link, regarded as a homogeneous space Γ\G, and the analytic structure on the surface singularity. In [59] Dolgachev mentions that F. Ehlers and J. Scherk pointed out to him that the diffeomorphisms that he obtains (and that we explained above) between the links of normal Gorenstein surface singularities with C∗ -action and quotient spaces of the form Γ\G, for G = SU (2),
SL(2, R) or N , also preserves the CR structures: on the link M it is determined by the analytic structure on V ; on Γ\G it is induced by the unique G-invariant CR structure on G [51]. I presume the same statement holds for all quotients Γ\G for the remaining 3-dimensional Lie groups.

Chapter IV

Within the Realm of the General Index Theorem The goal of this chapter is to show some of the ways in which the general index theorem of Atiyah-Singer has had impact in singularities theory. Mostly, I will restrict myself to some interesting applications of two “particular” cases of the index theorem: the Hirzebruch-Riemann-Roch formula and the Hirzebruch signature theorem. We consider also Rochlin’s signature theorem. The general philosophy is the following. The index theorem of Atiyah-Singer may be thought of as a beautiful and far-reaching generalization of the HirzebruchRiemann-Roch theorem, both in statement and in the spirit of the original proof (modified in [20, 21] to allow certain generalizations). Given a closed, oriented manifold M , vector bundles E and F over M and an elliptic operator D from the sections of E to those of F , one has that both the kernel and the cokernel of D are finite-dimensional, and the difference of these dimensions is by definition the analytic index of D. The index theorem gives a description of this integer in terms of topological data implicit in the elliptic operator, the so-called topological index. This establishes a very deep connection between analysis/geometry and topology. Special cases are the signature theorem of Hirzebruch, the Hirzebruch-Riemann-Roch theorem, the Lefschetz fixed point formula, the relation of the Dirac  operator with the A-genus for spin manifolds and several other fundamental theorems in mathematics. Rochlin’s signature theorem can also be seen through index theory (see §3 below). In the case of a compact manifold M with non-empty boundary ∂M , both sides of the index formula are still defined, the analytic index and its topological counterpart, but their difference need not vanish. This led Hirzebruch to introduce the signature defect, which has had significant applications to singularities. For elliptic operators in general, the signature defect was the starting point of a series of articles by Atiyah, Patodi and Singer, leading them to the definition of the

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η-invariant, which is the boundary contribution that gives the correction term for the index formula. This has been used in singularity theory in essentially two related general ways: (i) If (V, 0) is an isolated (real or complex) singularity germ in some affine space, then the diffeomorphism type of its link M = V ∩ Sε depends only on the analytic structure of V , and not on the choices of defining equations for V or the radius of the sphere (see Chapter I). Thus any invariant of closed manifolds gives automatically an invariant of singularities. This has led to a vast number of interesting results for complex singularities, especially in dimension 2. For example for the Rochlin invariant, the signature defect (we discuss both of these below), and in many more directions. Of course, the index theorem comes from the 1960s and since then there has been a tremendous evolution in mathematics, particularly in the low dimensions, where the theorems of Donaldson, Witten and others have given rise to powerful invariants of 3- and 4-dimensional manifolds, which are nowadays giving important information about surface singularities (see the last section of this chapter). It is worth noticing that (to my knowledge), essentially nothing has been done in this direction for real analytic singularities, and I believe this can be interesting. (ii) Consider now an isolated (real or complex) singularity germ (V, 0) and a resolution of this singularity π : V → V (see Section 5 below); these can be considered as manifolds with boundary the link M , and we can use them to compute the invariants associated to the link as in i) above. Furthermore, if (V, 0) is a complex analytic complete intersection germ, or more generally a smoothable singularity (see below), one also has a Milnor fibre F (or a smoothing) of V . These can also be considered as manifolds with boundary the link, and the same invariants that we evaluated using a resolution can be computed via the smoothing. This leads to interesting relations among the invariants of (V, 0) coming from a resolution and those coming from the smoothings. For instance the formula of Laufer (and its generalization to higher dimensions by Looigenga) for the Milnor number, and that of Durfee for the signature, are both obtained in this way.

IV.1

A review of characteristic classes

The material in this section is all standard, see for instance [170, 166, 242]. Let v = (v1 , . . . , vn ) be a vector field in an open set U ⊂ Rn ; the vector field is said to be continuous, smooth, analytic, etc., according as its components {v1 , . . . , vn } are continuous, smooth, analytic, etc., respectively. A singularity P of v is a point where all of its components vanish, i.e., vi (P ) = 0 for all i = 1, . . . , n, and a singularity of v is isolated if at every point x near P there is at least one component of v which is not zero. Let v be a vector field as above and let P be an isolated singularity of v. Let Sε be a small sphere around P . Then the Poincar´e-Hopf index of v at P is

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93

v the degree of the map ||v|| from Sε into the unit sphere in Rn . It is clear that all these definitions extend to vector fields on smooth manifolds, and a fundamental property of this index is the following theorem:

1.1 Theorem. (Poincar´e-Hopf ) Let M be a closed, oriented n-manifold and let v be a continuous vector field on M with isolated singularities. Define the total index of v, denoted P H(v, M ), to be the sum of all its local indices at the singular points. Then one has P H(v, M ) = χ(M ) , independently of v, where χ(M ) is the Euler characteristic. The proof of this theorem can be found in many text books. If M is now an oriented manifold with boundary, one has a similar theorem: 1.2 Theorem. Let M be a compact, oriented n-manifold with boundary ∂M , and let v be a non-singular vector field on a neighborhood U of ∂M . Then: (i) v can be extended to the interior of M with isolated singularities. (ii) the total index of v in M is independent of the way we extend it to the interior of M . In other words, the total index of v is fully determined by its behavior near the boundary. (iii) if v is everywhere transversal to the boundary and pointing outwards from M , then one has P H(v, M ) = χ(M ). If v is everywhere transversal to ∂M but it points inwards M , then P H(v, M ) = χ(M ) − χ(∂M ). There is another way of defining the index, which brings us closer to the topic that we discuss in a moment. Suppose first that M has no boundary and we want to construct a non-singular vector field on M , i.e., a non-zero section v of its tangent bundle T M . We do it step by step: let M j be the j-skeleton of M for some triangulation (or cell decomposition). It is clear that we can always construct a non-zero vector field on M 0 , which consists of isolated points. Now observe that if we have the vector field defined and non-zero on the boundary of a p-simplex σ, then the vector field can be regarded locally as a map ∂σ ∼ = Sp−1 → Sn−1 ; hence it extends to the interior of σ if p < n. In this way we see that a never-zero vector field can be constructed up to the (n − 1)-skeleton of M . When we try to extend it to the n-skeleton, for each n-simplex σ we have the vector field on its boundary, a topological (n− 1)-sphere, defining an element in the homotopy group πn−1 (§n−1 ) ∼ = Z. The vector field can be extended to this cell iff the corresponding map is nulhomotopic. Thus we have for each n-simplex a local obstruction in Z. In fact the vector field always extends to all of σ minus its barycentre σ  and the corresponding local obstruction in Z is the Poincar´e-Hopf index at σ . This gives rise to a cochain of dimension n, which is in fact a cocycle, hence representing a cohomology class with integer (local) coefficients. The cohomology class that we obtain in this way is the Euler class of M , e(M ) ∈ H n (M ; Z), which evaluated on the orientation cycle [M ] gives a number: the Euler characteristic χ(M ).

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Suppose now that M has boundary ∂M and the vector field v is defined and non-singular around the boundary. The previous stepwise process tells us that v can be extended to all of ∂M ∪ M (n−1) . For each n-cell of M we have a local obstruction in Z. Thus we get a cocycle e(M, v) in the relative cohomology group H n (M, ∂M ; Z). As a relative class e(M, v) does depend on the choice of the vector field, generally speaking, though its image in H n (M ; Z) depends only on M . This is the Euler class of M relative to v. Evaluating e(M, v) on the orientation cycle of (M, ∂M ) we get a number, the total index of v in M . Let us now extend these constructions to define the Chern classes via obstruction theory. This can be done with no extra work for a continuous complex vector bundle ξ of complex (fibre) dimension n over a finite simplicial complex K of real dimension 2m; we assume for simplicity n ≤ 2m, but this is not necessary. We denote by K j the j th -skeleton. 1.3 Definition. Consider a set F (r) = {v1 , . . . , vr } of r continuous sections of ξ defined on a subcomplex A of K. If the vectors vi are linearly independent over C at every point of A, we say that F (r) defines an r-frame, or a non-singular r-field on A. If at a point a ∈ A one (at least) of the vectors vi is zero, or if the r vectors are not linearly independent over C, then we say that a is a singular point of the r-field F (r) . Let Wr (n) be the Stiefel manifold of complex r-frames in Cn . We know (see [242]) that this manifold is (2n−2r)-connected and its first non-zero homotopy group is π2n−2r+1 (Wr (n)) ∼ = Z. The bundle of r-frames of ξ, denoted by Wr (ξ), is the bundle associated with ξ whose fibre over x ∈ K is the set of r-frames in ξx . The Chern class cq (ξ) ∈ H 2q (K) is the first possibly non-zero obstruction to construct a section of Wr (ξ). Let us construct this class using the same stepwise process we used to construct the Euler class of a manifold. Let σ be a k-simplex of K (or a k-cell of a given cell decomposition of a smooth manifold). If the section F (r) of Wr (ξ) is already defined over the boundary of σ, it gives a map: (r)

pr2 F ∂σ ∼ = σ × Wr (n) −−−−→ Wr (n), = Sk−1 −−−−→ Wr (ξ)|σ ∼

thus an element of πk−1 (Wr (n)). If k ≤ 2n − 2r + 1, this homotopy group is zero, so the section F (r) can be extended to σ without singularities. If k = 2n − 2r + 2 = 2q, we meet an obstruction. So we can always construct a section F (r) of Wr (ξ) over the (2q − 1)-skeleton. When we try to extend F (r) to the 2q-skeleton, we have for each simplex an r-frame on its boundary, which defines an element I(F (r) , σ) ∈ π2q−1 (Wr (n)) ∼ = Z. Let us define a cochain γ ∈ C 2q (K; π2q−1 (Wr (n))) (r) by γ(σ) = I(F , σ) for each simplex σ, and then extend it by linearity. This cochain is actually a cocycle and the cohomology class that it represents is the q th Chern class cq (ξ) of ξ in H 2q (K; Z). It is independent of the various choices involved in its definition. We have one such class for each q = 1, . . . , n. For an almost complex manifold M 2m , its Chern classes are those of its tangent bundle. Note that cm (M ) coincides with the Euler class of the underlying real tangent bundle TR M .

IV.1. A review of characteristic classes

95

Suppose now that L is a sub-complex of K such that the bundle ξ is trivial over L, and we are given a specific trivialization τ of this bundle. Then we can  = K/L; this is a finite CW-complex. The trivialization form the quotient space K n τ : ξ|L → L × C gives us a way to identify all the fibres of ξ over L, and so we  The Chern classes get a complex vector bundle ξ over K.  ∈ H 2i (K;  Z) ∼ ci (ξ) = H 2i (K, L; Z) , i = 1, . . . , n , of ξ are by definition the Chern classes of ξ relative to the trivialization τ over L, denoted ci (ξ; τ ). These classes do depend on the choice of τ , generally speaking, but their images in H ∗ (K; Z) are the usual Chern classes, which depend only on ξ and K. Let us now repeat the same constructions as before, but we consider now an oriented real vector bundle ζ of fibre dimension n over a finite simplicial complex K of dimension m; for simplicity we assume n ≤ m. The previous arguments tell us that the obstruction for constructing a non-zero section of ζ is a cohomology class e(ζ) ∈ H n (K; Z), which is (by definition) the Euler class of ζ. The image of e(ζ) in H n (K; Z2 ), denoted wn (ζ), is by definition the top-dimensional StiefelWhitney class of ζ. Notice that the orientation of ζ is only necessary to define the Euler class, which has integral coefficients. The Stiefel-Whitney class wn (ζ) is well defined even for non-orientable bundles. Now let us mimic the previous construction of Chern classes: we want to construct at each point of K two linearly independent sections of ζ. This means that (up to homotopy) we want a section of the bundle V2 (ζ), whose fibre at each point is the Stiefel manifold V2 (n) of oriented 2-frames in Rn . The homotopy groups of V2 (n) vanish up to dimension i = n − 3 and one has πn−2 (V2 (n)) ∼ =Z for n even, and πn−2 (V2 (n)) ∼ = Z2 for n odd, see [244, 25.6]. Thus, for n even, one gets that the primary obstruction Wn−1 (ζ) for constructing a 2-frame of ζ lives in H n−1 (K; πn−2 (V2 (n)) ∼ = H n−1 (K; Z), while for n odd the obstruction is a class wn−1 (ζ) in H n−1 (K; Z2 ). For n even, let us denote also by wn−1 (ζ) ∈ H n−1 (K; Z2 ) the reduction modulo 2 of Wn−1 (ζ). By definition, the class wn−1 (ζ) is the (n−1)th Stiefel-Whitney class of ζ, for all n. More generally, if we try to construct a k-frame of ζ by the stepwise process, the first possibly non-zero obstruction will be a cohomology class of K in dimension (n − k + 1) with coefficients in πn−k (Vk (n)), which is isomorphic to Z when n − k is even, and it is isomorphic to Z2 when n − k is odd and k > 1, see [244, 25.6]. In all cases we can consider the corresponding class wi (ζ) ∈ H n−k+1 (K; Z2 ); this is by definition the (i)th Stiefel-Whitney class of ζ, i = n − k + 1 = 1, . . . , n. We notice that by construction, when n − k is even, or when k = 1, the corresponding Stiefel-Whitney class automatically has a lifting to the integral homology. However, for n − k odd, such a lifting may not exist. For a smooth manifold, its Stiefel-Whitney classes are those of its tangent bundle. Of course just as for Chern classes, when one has a trivialization of the bundle ζ over a subcomplex L ⊂ K, one also has a theory of relative Stiefel-Whitney

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classes, which live in H ∗ (K, L; Z2 ). Their images in the absolute cohomology are the usual Stiefel-Whitney classes, which depend only on the space K and the vector bundle ζ, while the relative classes do depend in general on the choice of trivialization of ζ over L. There is finally a third type of characteristic classes associated to vector bundles, which is very useful and important in order to obtain further information of the bundles. These are the Pontryagin classes. To define these, we start with a real vector bundle ζ over K as before. For each fibre Vx of ζ consider the complexification Vx ⊗ C = Vx ⊗R C. The union of all these vectors gives a new vector bundle over K, which is now a complex vector bundle that we denote by ζ ⊗ C and call it the complexification of ζ. Notice that the underlying real vector bundle (ζ ⊗ C)R is canonically isomorphic to the Whitney sum ζ ⊕ ζ, with each fibre being canonically of the form Vx ⊕ i Vx . We now consider the Chern classes of the complex bundle ζ ⊗ C. It is an exercise (see [170, §15)] to show that the odd-dimensional classes c2i+1 (ζ ⊗ C) are all elements of order 2, i.e., twice the element is zero in cohomology. Ignoring these elements of order 2, the ith Pontryagin class of ζ, pi (ζ) ∈ H 4i (K; Z), is defined to be the integral class (−1)i c2i (ζ ⊗ C), the sign being just a useful convention. There is a situation which is especially relevant for what follows in this chapter, that is when the bundle we start with is already a complex bundle, particularly the tangent bundle of a complex manifold. So we consider now a complex vector bundle ξ over K; we may forget the complex structure on ξ and regard the underlying real vector bundle ξR . One has (see [170, 15.4]) that the complexification of this bundle is canonically isomorphic to the Whitney sum ξ ⊕ ξ, where ξ is the dual vector bundle. We also know that the Chern classes satisfy the Whitney product formula with respect to Whitney sums. This means that one has: 1 − p1 (ξ) + − · · · ± pn (ξ) = (1 + c1 (ξ) + · · · + cn (ξ)) · (1 + c1 (ξ) + · · · + cn (ξ)) and the Chern classes of ξ satisfy: ci (ξ) = (−1)i ci (ξ), see [170, 14.9]. Thus one has ([170, 15.5]): 1.4 Proposition. For any complex n-plane bundle ξ, the Chern classes of ξ determine the Pontryagin classes of ξR by the formula (dropping ξ from the notation for simplicity): 1 − p1 + p2 − + · · · ± pn = (1 − c1 + c2 − + · · · ± cn ) · (1 + c1 + c2 + · · · + cn ) and therefore pi (ξR ) is:

pi = c2i − 2ci−1 ci+1 + − · · · ± 2c1 c2i−1 ∓ 2c2i .

In particular, for a complex manifold M of dimension 2 one has: p1 (M ) = c1 (M )2 − 2c2 (M ). 1.5 Remark. Notice that we have only considered here characteristic classes of vector bundles and manifolds. These will be used later to study invariants of

IV.2. The signature and Riemann Roch theorems

97

singular spaces, usually by looking at the characteristic classes of either a resolution of the singular variety or a smoothing of it (see the definitions below). These are useful invariants, however they are not intrinsic to the singular variety and depend on the choice of resolution or smoothing. There is a vast literature about “Chern classes for singular varities”, which are intrinsic to varieties. This began with the work of M.H. Schwartz [222] and R. MacPherson [152]; we refer to [34] for an overview of the topic. But we shall not use these in the sequel.

IV.2

On Hirzebruch’s theorems about the signature and Riemann-Roch

Let M be a compact, smooth and oriented manifold (possibly with non-empty boundary ∂M ) of dimension 2n ≥ 2 and consider its middle cohomology with real coefficients, H n (M, ∂M ; R). This is a finite-dimensional vector space over which one has a non-degenerate symmetric bilinear form: bM : H n (M, ∂M ; R) × H n (M, ∂M ; R) −→ H 2n (M, ∂M ; R) ∼ = R given by the cup product. The Poincar´e-Lefschetz duality isomorphism implies that this pairing is non-degenerate. This duality isomorphism also allows us to look at this bilinear pairing in homology, b∗M : Hn (M ; R) × Hn (M ; R) −→ H0 (M ; R) ∼ = R, where it is given by the intersection product of cycles. One usually refers to the corresponding quadratic form in H n (M, ∂M ; R) as the quadratic form of M . 2.1 Definition. Let n = 2k. The signature of M 4k , σ(M ), is the signature of the bilinear form bM . That is, σ(M ) = s+ − s− , the number s+ of positive eigenvalues, minus the number s− of negative eigenvalues of bM . For a manifold of dimension 4k+2 the signature is defined to be 0. It is clear that one has σ(M ) = −σ(−M ) where −M is M with reversed orientation. One has: 2.2 Theorem. (R. Thom [250].) The signature satisfies: (i) it is an invariant of oriented cobordism, in particular it vanishes when the manifold is an oriented boundary; (ii) it is multiplicative, i.e., σ(M 4k × N 4r ) = σ(M 4k ) · σ(N 4r ); (iii) it is additive under disjoint unions: σ(M 4k  N 4r ) = σ(M 4k ) + σ(N 4r ).

98

Chapter IV. The General Index Theorem

Thom also proved in [250, IV.14, p. 81] that for k = 1 one has σ(M ) =

1 p1 (M )[M ] , 3

a third of the Pontryagin number. This gives a clue to what is coming next. To give a precise statement of the signature theorem of Hirzebruch we need to introduce the multiplicative sequences of polynomials (we follow [101]). These are sequences {Kj } of polynomials in the indeterminates p1 , p2 , . . . . One sets p0 = 1 and considers the ring Q[p1 , p2 , . . . ]. This ring is graded by assigning weight i to each variable pi and weight i1 + i2 + · · · + in to the product pi1 pi2 · · · pin . Each polynomial Kj is homogeneous of weight j. The sequence {Kj } is called multiplicative if every identity of the form: 1 + p1 z + p2 z 2 + · · · = (1 + p1 z + p2 z 2 + · · · ) · (1 + p1 z + p2 z 2 + · · · ) with z, pi , pi , pi indeterminate, implies an identity ∞ 

i

Ki (p1 , . . . , pi )z =

 ∞

i=0

Ki (p1 , . . . , pi )z i

i=0

 ∞

 Kj (p1 , . . . , pj )z . j

j=0

Given one such sequence, the power series, Q(z) = K(1 + z) =

∞ 

bi z i ,

b0 = 1, bi = Ki (1, 0, . . . , 0) ∈ Q ,

i=0

is called the characteristic power series of the multiplicative sequence. It turns out [101, 1.2.1] that each multiplicative sequence of polynomials as above is completely determined by its characteristicpower series Q(z). Furthermore ([101, ∞ 1.2.2]), every formal power series Q(z) = i=0 bi z i , b0 = 1, bi ∈ Q , corresponds to a multiplicative sequence of polynomials. Hence, to describe such a sequence we only need to specify the corresponding power series Q(z). So now consider the power series Q(z) =

√ ∞  z 22r √ = 1+ Br z r . (−1)r−1 (2r)! tanh z r=0

The coefficients Br are the Bernoulli numbers. Hirzebruch in [101] gives the first of these coefficients, some of them are: B1 =

1 , 6

B2 =

1 , 30

B3 =

1 42

,...,

B8 =

3617 . 510

2.3 Definition. The multiplicative sequence corresponding to this power series is the L-sequence of Hirzebruch, denoted {Lj (p1 , . . . , pj )}.

IV.2. The signature and Riemann Roch theorems

99

The first of these polynomials are listed by Hirzebruch. These are: L1 = 13 p1 , 1 (7p2 − p21 ) , L2 = 45 1 (62p3 − 13p2 p1 + 2p31 ). L3 = 33 ·5·7 We can now state the signature theorem of Hirzebruch [101, Thm. 8.2.2]: 2.4 Theorem. Assume M is closed (i.e., ∂M = ∅) of dimension 4k > 0. Then its signature σ(M ) is:   σ(M ) = Lk p1 (M ), . . . , pk (M ) [M ] , where the pi (M ) ∈ H 4i (M ; Z) are the Pontryagin classes of M . The polynomial on the right-hand side is called the L-genus of the manifold M , denoted L[M ]. Notice that just the mere fact this genus is an integer is already a miracle. The proof of this theorem was given by Hirzebruch in [101], and as mentioned before, this can also be deduced directly from the general Index Theorem of AtiyahSinger. The idea of the proof is the following (see [101, Chapter II] or [170, p. 225]). First one notices that by construction, the L-genus satisfies a multiplicative property similar to that satisfied by the signature (see Definition 2.3 above), and they are both additive under disjoint unions, which is the operation in cobordism. From this, together with the fact that the Pontryagin numbers are invariants of oriented cobordism, one deduces that the correspondences M → σ(M ) and M → L[M ] give rise to algebra homomorphisms Ω∗ ⊗ Q −→ Q, where Ω∗ is the oriented cobordism ring. Hence it is enough to prove Theorem 2.4 for a set of generators of the algebra Ω∗ ⊗ Q, which vanishes in dimensions ≡ 0 mod 4 (by [250]). Such a set is provided by the complex projective spaces CP2k . It is not hard to show that the signature of CP2k is always +1. So one needs to show only that the L-genus of every complex projective space is also +1, and this is proved in [101, p. 225] or [170]. Let us restrict now to dimension 4. As mentioned above, in this case the theorem was given by R. Thom [250, IV.14, p. 81] and it says: σ(M ) =

p1 (M )[M ] . 3

(2.5)

If we now take M to be a complex manifold (even almost complex is enough), then we know that p1 = c21 − 2c2 , and so the previous formula takes the very nice shape: K 2 − 2 χ(M ) , (2.6) σ(M ) = 3 where K is the canonical divisor, representing a homology class which is the Poincar´e dual of the first Chern class of the cotangent bundle T ∗ M , and χ(M ) is the topological Euler characteristic.

100

Chapter IV. The General Index Theorem

Let us move now towards the Hirzebruch-Riemann-Roch theorem. This begins with Riemann’s calculation that the holomorphic differentials on a compact Riemann surface S of genus g ≥ 0 is a complex vector space of dimension g, i.e., dim H 0 (S; KS ) = g , where KS is the cotangent (or canonical) bundle on S. The next step is to consider an arbitrary holomorphic line bundle L over S, then we would like to be able to determine the dimension of the space of global sections of it H 0 (S; L), which we denote h0 (L) for simplicity; in particular one may ask whether this number is still determined by the topology. Before stating the theorem, notice that the above formula can be written as: h0 (OS ) − h0 (KS ) =

1 1 (2 − 2g) = c1 (S)[S] 2 2

where h0 (OS ) = 1 are the holomorphic functions from S to C and c1 is the Chern class of T S. The Riemann-Roch formula for line bundles over a Riemann surface in general says:   1 0 0 −1 h (L) − h (KS ⊗ L ) = c1 (L) + c1 (S) [S]. 2 The Serre duality theorem for line bundles over Riemann surfaces gives the duality: H 0 (KS ⊗ L−1 ) ∼ = H 1 (L) , and therefore the Riemann-Roch formula can be expressed as:   1 h0 (L) − h1 (L) = c1 (L) + c1 (S) [S]. 2 That is, the analytic Euler characteristic χ(L) =

1 

(−1)i hi (L) ,

i=0

is determined by the topological invariants on the right-hand side. We write this as:   χ(S; L) = T d1 (S) + c1 (L) [S] , (2.7) where T d1 (S) = 12 c1 (S). This is the expression that generalizes to higher dimensions. In order to explain this we need to introduce another multiplicative sequence of polynomials, the Todd sequence {T dk }. This is determined by the power series: ∞

Q(x) =

 x Bk 2k 1 x . = 1+ x+ (−1)k−1 −x 1−e 2 (2k)! k=0

IV.2. The signature and Riemann Roch theorems

101

The first polynomials in this sequence are [101]: T d1 =

1 c1 , 2

T d4 =

T d2 = 1 720 (−c4

1 (c2 + c21 ) , 12

T d3 =

1 c2 c1 , 24

+ c3 c1 + 3c22 + 4c2 c21 − c41 ).

2.8 Definition. Let M be an almost complex manifold of real dimension 2k. Then its Todd genus T d(M ) is: T d(M ) = T dk (c1 (M ), . . . , ck (M ))[M ] , where ci (M ) ∈ H 2i (M ; Z), i = 1, . . . , k, are the Chern classes of M . 2.9 Definition. Let M be a complex manifold of dimension k. Its analytic Euler characteristic (also called the arithmetic genus) is: χ(M ; O) =

k 

(−1)i hi (M ; O) ;

i=0

and more generally, if L is a holomorphic bundle over M , then the analytic Euler characteristic of L is: χ(M ; L) =

k 

(−1)i hi (M ; L) .

i=0

Now we can state the simplest version of the Hirzebruch-Riemann-Roch theorem: 2.10 Theorem. Let M be a complex manifold. Then χ(M ; O) = T d(M ) . For algebraic manifolds of dimension 2, this is Noether’s formula. Stated in this way, Theorem 2.10 only makes sense for complex manifolds and Hirzebruch’s proof was only for algebraic manifolds. However one can re-phrase it using the ¯ index theorem, replacing the left-hand side by the index of the ∂-operator, and the theorem holds for almost complex manifolds. This implies in particular that T d(M ) is an integer, which is another miracle. In Section 4 below we give a theorem [76] which shows that for complex manifolds, the parity of T d(M ) is detected by a certain “mod (2) index”, which for complex surfaces is the Arf invariant in Rochlin’s theorem, which we discuss in the following section. More generally, given a holomorphic bundle L, the Riemann-Roch formula says that the analytic Euler characteristic of L is given by the Todd genus of M and a polynomial in the Chern class c1 (L) and the Chern classes of M .

102

Chapter IV. The General Index Theorem

Making c = c1 (L), in the low dimensions this takes the form:   1 χ(M 1 ; L) = c + c1 (M ) [M ] , 2 1   1 2 2 c + c · c1 (M ) + c2 (M ) + c21 (M ) [M ] , χ(M ; L) = 2 12 1  1 2 1 1 3 3 c + c c1 (M ) + c (c2 (M ) + c21 (M )) + c1 (M )c2 (M ) [M ] . χ(M ; L) = 6 4 12 24 The general Riemann-Roch theorem (see [101, Ch. 4] and [21, §4]) expresses the analytic Euler characteristic χ(M ; E) of every holomorphic vector bundle over M (E of any dimension) in terms of the total Todd class of M (i.e., the collection of all cohomology classes in all dimensions, obtained by evaluating the Todd sequence of polynomials in the Chern classes of M ), and the Chern character of the bundle (see [101, §10]), which is a class in the cohomology ring H ∗ (M ; Q).

IV.3

Spin and Spinc structures on 4-manifolds. Rochlin’s theorem

We refer to [124] for a clear account of spin and spinc structures on manifolds. The group Spin(n), n > 1, is usually defined as a subgroup of the group of units of an appropriate (Clifford) algebra, but it can be equivalently defined as the non-trivial double cover of SO(n); since for n > 2 the fundamental group of SO(n) is Z/2, it follows that in these dimensions Spin(n) is also the universal covering group of SO(n). In low dimensions one has that Spin(2) is the circle S1 regarded as the 2fold cover Spin(2) → SO(2); Spin(3) is the 3-sphere, isomorphic to SU (2) ∼ = Sp(1); Spin(4) is SU (2) × SU (2) , since SO(4) ∼ = SO(3) × SU (2). Now, given a closed, connected smooth manifold M of dimension n, the structure group of its tangent bundle T M is GL(n, R). To have a reduction of the structure group of this bundle to a certain subgroup G ⊂ GL(n, R) means that we have a smooth atlas {(Ui , φi )} for M , so that T M |Ui is trivial on each chart, and the gluing functions for T M on each chart, φij : Ui ∩ Uj → GL(n, R) can be all taken in G (c.f. Ch. III, §6). Since each basis of Rn is homotopic to an orthonormal one, the group GL(n, R) has O(n) as a deformation retract. This means that endowing M with a Riemannian metric, we can reduce the structure group of T M from GL(n, R) to O(n). Equivalently, the bundle T M is classified, up to isomorphism, by a continuous map (see [164]) τM : T M → BGL(n, R) into the classifying space of GL(n, R). There is a map induced by the inclusion ι BO(n)
→ BGL(n, R), and a reduction of the structure group of T M to O(n)

IV.3. Rochlin’s theorem for Spinc manifolds

103

simply means a lifting of the classifying map τM from BGL(n, R) to BO(n). If M is an orientable manifold, we may reduce the structure group of the bundle T M further to SO(n). This means choosing our atlas so that all the gluing maps have determinant 1, i.e., we are fixing an orientation on M , or equivalently, lifting the classifying map τM further to BSO(n). We remark that such lifting of τM exists only because we are assuming M is orientable, and there are exactly two homotopy classes of such liftings, corresponding to the possible orientations on M . It is an exercise (see [124]) to show that a smooth compact manifold M is orientable iff its first Stiefel-Whitney class w1 (M ) ∈ H1 (M ; Z2 ) vanishes. The covering map Spin(n) → SO(n) induces a map of classifying spaces B Spin(n) → BSO(n), and we may ask to further lift τM from BSO(n) to B Spin(n). 3.1 Definition. A compact, oriented manifold M admits a spin structure if the structure group of its tangent bundle can be lifted from SO(n) to Spin(n); the various homotopy classes of such liftings are the different spin structures on M . In this case one says, shortly, that M is a spin manifold. Alternatively, an orientation on M determines a principal SO(n)-bundle P over M , whose fibre at each point x ∈ M is the set of all orthonormal, positively oriented basis of the corresponding tangent space Tx M ; and a spin structure on M means a principal spin bundle P over M which double covers P and at each x ∈ M restricts to the double cover Spin(n) → SO(n). One has (see [124, 1.7]): 3.2 Proposition. A compact smooth manifold M admits a spin structure iff its Stiefel-Whitney classes w1 (M ) and w2 (M ) vanish; and if this happens, then the different spin structures on M are classified by H 1 (M ; Z2 ). Thus, for instance, every parallelizable manifold is spin, since T M trivial implies we can reduce its structure group to 1, and all its Stiefel-Whitney classes vanish. Also notice that every homology sphere (of dimension > 2) admits a unique spin structure. Let us assume now that M is spin. Then one can easily see that its quadratic form is automatically an even form, and therefore its signature must be divisible by 4. A general result in algebra tells us that it is actually divisible by 8, which is already a non-trivial fact. Rochlin’s remarkable theorem in 1952 says more: σ(M ) ≡ 0

mod (16) .

(3.3)

This was a key result in 4-manifold theory, being closely related to other peculiarities of low-dimensional topology and triangulation theory. This was generalized by M. Kervaire and J. Milnor for oriented 4-manifolds which may not be spin provided there is an embedded 2-sphere S in M representing an integral homology class whose reduction modulo 2 is the Stiefel-Whitney class. In this situation they proved: (3.4) σ(M ) ≡ S 2 mod (16)

104

Chapter IV. The General Index Theorem

where S 2 is the self-intersection of S in M . In order to say more about this result of Kervaire-Milnor, and its later generalization by Rochlin himself, it is convenient to speak about spinc -structures on manifolds. For this we recall that the oriented, differentiable 2-plane vector bundles over an oriented manifold are in 1-to-1 correspondence with the cohomology group H 2 (M ; Z), the correspondence being given by associating to each such bundle its Euler class. Notice that SO(2) ∼ = U (1), so we can identify oriented 2-plane bundles with complex line bundles. We also recall that Spin(n)c is defined to be the subgroup of Spin(n+ 2) that double covers the group SO(n)× SO(2) ⊂ SO(n+ 2). One has an isomorphism Spin(n)c ∼ = Spin(n) ×Z2 SO(2) , and a short exact sequence 0 −→ Z2 −→ Spin(n)c −→ SO(n) × SO(2) −→ 1 , where Z2 ⊂ Spin(n)c is generated by the element [(−1, 1)] = [(1, −1)]. 3.5 Definition. An oriented n-manifold M admits a spinc structure if there exists a complex line bundle L over M , called the determinant bundle for the spinc structure, such that the structure group of T M ⊕ L lifts to Spin(n)c . One has that M admits a spinc structure iff there exists an integral homology class W ∈ Hn−2 (M ; Z) whose reduction modulo 2 is the Poincar´e dual of w2 (M ), the 2nd Stiefel-Whitney class of M (see for instance [124, Theorem D.2, p. 391]. This class W is precisely the Chern class of the determinant bundle L. It follows that if M admits a spinc structure, then all other spinc structures correspond bijectively with the elements in H 2 (M ; Z), which can be regarded as the group of C∞ complex line bundles over M . Of course if M is a complex manifold, then it has a canonical spinc structure given by −K, the anti-canonical class, which is the 2Chern class of its tangent T M . The canonical bunbundle, which equals that of the complex line bundle  dle KM = 2 T ∗ M gives another spinc structure. Also every spin manifold is canonically spinc with the trivial bundle as determinant (and W = ∅). 3.6 Definition. An oriented submanifold W ⊂ M of codimension 2 is characteristic if it is the Chern class of a line bundle over M which is the determinant of some spinc structure on M . Of course this is equivalent to saying that W represents an integral homology class W ∈ Hn−2 (M ; Z) whose reduction modulo 2 is the Poincar´e dual of w2 (M ), the 2nd Stiefel-Whitney class of M , which is the usual definition of a characteristic submanifold. The following generalization of the theorems of Rochlin (3.3) and KervaireMilnor (3.4) was given by Rochlin himself in [209]. An essentially complete proof can be found in [81]; a generalization to higher dimensions was given in [192].

IV.4. Spinc structures on complex surfaces

105

3.9 Theorem. Let M be an oriented closed 4-manifold and W ⊂ M a characteristic submanifold, then one has: σ(M ) − W 2 ≡ 8 Arf(W )

mod (16) ,

where W 2 is the self-intersection number of W and Arf(W ) ∈ {0, 1} is the Arf invariant of a certain quadratic form on H1 (W ; Z2 ). We leave it as an exercise for the reader to prove that every closed, oriented 4-manifold admits a spinc structure. We recall that the Arf invariant of a quadratic form on a finite-dimensional vector space over Z2 takes values in {0, 1} and can be defined to be 0 iff it takes the value 0 more times than it takes the value 1; two such quadratic forms are equivalent iff they have the same Arf invariant (see the appendix in [211] for details). Of course if M is spin we can take W = ∅ and we recover (3.3). And if M is not spin but we can take W to be a 2-sphere, then H1 (W ; Z2 ) = 0, so Arf(W ) = 0 and we recover (3.4).

IV.4

Spin and Spinc structures on complex surfaces. Rochlin’s theorem

We now discuss Spin and Spinc structures on complex surfaces and give a variant of Rochlin’s Theorem 3.1 when both M and W are complex analytic. Our approach is via algebraic geometry, and in doing so one is naturally led to considering divisors on M , rather than smooth submanifolds. But the definition of the Arf invariant in Rochlin’s theorem only makes sense when W is smooth. Thus we were led in [76] to considering another mod (2) invariant, instead of Arf(W ), that comes naturally from algebraic geometry. This invariant is an extension for divisors of the mod (2) index introduced in [16, 19], which is the mod (2) index of a Dirac operator. Essentially everything we say here is taken from [76] and holds for all complex dimensions of the form 4k + 2; here we restrict to complex dimension 2 for simplicity. We recall (c.f. III.6) that if M is a compact complex manifold of dimension 2, then its canonical bundle K = KM , is the bundle of holomorphic 2-forms on M , −1 whose Chern class is the negative of c1 (T M ). The anti-canonical bundle KM is the 2 c T M . This bundle determines a canonical spin structure on exterior product −1 M , for which KM is the determinant bundle (c.f. [192]). We denote by K the divisor of K, the canonical divisor of M ; −K is then the anti-canonical divisor. Notice that −K represents the Poincar´e dual of the Chern class c1 (K) ∈ H 2 (M ; Z). Since for complex manifolds the Chern classes reduced modulo 2 give the corresponding Stiefel-Whitney classes (see Section 1), it follows that both ±K represent integral homology classes whose reduction modulo 2 is the second Stiefel-Whitney class of M . However in most cases they are singular, non-reduced and reducible, so they cannot be used as characteristic submanifolds for M in the sense of Section 3. Yet

106

Chapter IV. The General Index Theorem

we note that a submanifold W ⊂ M is characteristic iff it represents a homology class of the form 2D − K for some integral class D ∈ H 2 (M ; Z). This group classifies the different spinc structures on M , and it can be regarded also as the group of C ∞ complex line bundles over M . Given a divisor D, we can form a divisor W = 2D − K , which corresponds to the line bundle D2 ⊗ K−1 ; the reduction modulo 2 of the homology class represented by W is the second Stiefel-Whitney class of M . Summarizing, one has: 4.1 Proposition. Every complex manifold M has a canonical spinc structure whose determinant bundle is the anti-canonical divisor −K. Furthermore, every line bundle LW on M of the form D2 ⊗ K−1 determines canonically a spinc structure on M whose determinant bundle is LW . Notice that if W were a smooth submanifold of M , then W would be characteristic in Rochlin’s sense. This justifies the following definition: 4.2 Definition. A characteristic divisor of M is a non-negative divisor W ≥ 0 of M which can be expressed as W = 2D − K , where D is some (any) divisor of M and K is the canonical divisor. We now want to introduce the aforementioned mod (2) invariant of characteristic divisors that will replace Rochlin’s Arf invariant in our discussion. For this we recall (see [16, 3.2]) that on a Riemann surface X the spin structures correspond bijectively with the holomorphic square roots of the canonical bundle KX , that is with the (isomorphism classes of) holomorphic line bundles L over X with L2 ∼ = K, where L2 = L ⊗ L. These bundles are called theta characteristics on the manifold; they were first studied by Krazer (1903). In [16, 177] Atiyah and Mumford showed (independently) that for Riemann surfaces, the dimension of the space of sections of these bundles, dim H 0 (X, L), reduced modulo 2, is stable under holomorphic deformations. To explain this, Atiyah showed that the parity of dim H 0 (X, L) can be regarded as being the mod (2) index of the Dirac operator for the corresponding spin structure. Similarly, given a compact complex surface M as above, and an effective divisor W on M , one has the canonical (or dualizing) sheaf ωW . This exists for all effective divisors (possibly non-reduced, reducible) on complex manifolds, see for instance [24]. One may define: 4.3 Definition. A theta characteristic on W is the restriction to W of a holomorphic line bundle D on M , such that D2 |W ∼ = ωW . One has: 4.4 Proposition. Let W = 2D − K be a characteristic divisor of M , and let L = LW , D be the corresponding line bundles. Then D|W is a theta characteristic on W .

IV.4. Spinc structures on complex surfaces

107

Proof. We recall that for a Riemann surface E in M the classical adjunction formula says KE ∼ = KM |E ⊗ νE , where νE is the normal sheaf. Similarly, given an effective divisor W in M defined by a section of a holomorphic line bundle L ∼ = LW , the restriction of L to W can be identified with the normal sheaf OW (W ) of W , and one also has the adjunction formula (see [24]): ωW ∼ = KM |W ⊗ L|W . By hypothesis one has L ∼ = D2 ⊗ K−1 . Therefore: M

−1 ωW ∼ |W ∼ = KM |W ⊗ L|W ∼ = KM |W ⊗ D2 |W ⊗ KM = D2 |W .




Now define (following [16, 19, 177, 76]): 4.5 Definition. The mod 2 index of the characteristic divisor W = 2D − K is:  dim H 0 (W, D) mod 2 , if W = 0 , (W ) = 0 , if W = 0 ; i.e., for W = 0 it is the reduction modulo 2 of the space of sections of its associated theta characteristic D|W . 4.6 Remarks. (i) Notice that given the divisor W , the corresponding bundle D is well defined modulo the 2-torsion in the Picard group Pic M of holomorphic line bundles on M . However the Hirzebruch-Riemann-Roch formula [101] of §2 above, together with Theorem 4.7 below, grants that this invariant depends only on the numerical equivalence class of D and is independent of the 2-torsion in Pic M . (ii) We also remark that everything we have said so far holds for compact manifolds of complex dimension 4k + 2, with obvious adaptations. In this case the characteristic divisor has dimension 2k 4k + 1 and the corresponding mod 2 index is the reduction modulo 2 of i=0 dim H i (W, D), see [76, 1.4]. The following theorem also holds in this general setting ([76, 2.1]), but for simplicity we do it here only for k = 0. 4.7 Theorem. Let M be a compact complex surface, let W = 2D 2− K be a characteristic divisor of M , (W ) its mod 2 index, and let χ(M,D) = i=0 (−1)i hi (M,D) be the analytic Euler characteristic of M with coefficients in D (see [101]). Then one has: (W ) ≡ χ(M, D) mod 2 . Proof. Assume first that W = 0, and consider the exact sequence of sheaves over M : s∗

0 −→ KM ⊗ D−1 −→ D −→ D|W −→ 0 , r

108

Chapter IV. The General Index Theorem

where s∗ is multiplication by the section s of L that defines W and r is the restriction to W . One has the associated long exact sequence β

0 → H 0 (KM ⊗ D−1 ) → H 0 (D) → H 0 (D|W ) → H 1 (KM ⊗ D−1 ) → H 1 (D) → · · · . α

If we denote by hi (·) the dimension of the corresponding cohomology group, then we want to prove: h0 (W, D|W ) ≡

2 

hi (M, D)

mod 2

1=0

and by exactness of the sequence above we know: h0 (W, D|W ) ≡ h0 (M, KM ⊗ D−1 ) + h0 (M, D) + dim Im (α)

mod 2.

Serre’s duality on M gives: H 0 (M, KM ⊗ D−1 ) ∼ = H 2 (M, D) , so, again by exactness, the theorem will be proved if we show that dim Ker (β) ≡ h1 (M, D)

mod 2 ,

but Serre’s duality tells us that β is essentially defined by the cup-product in is  a skew-form, so it can H 1 (M, KM ⊗ D−1 ), and in dimension 2 the cup-product  0 b be expressed as a direct sum of blocks of the form , hence the theorem. −b 0 This proves Theorem 4.7 for W = 0 characteristic. It remains to prove the theorem for W = 0; this is left as an exercise (see [76] for details).
Let us now recall that the Riemann-Roch formula says that the analytic Euler characteristic of M with coefficients in D is given by: 1 2 (D − D · K) 2 1 1 (K 2 + χ(M )) + (W 2 − K 2 ) , = 12 8

χ(M, D) = T d(M ) +

since W = 2D − K, where χ(M ) is the usual (topological) Euler-Poincar´e characteristic of M . Thus Theorem 4.7 can be reformulated as: (W ) ≡

1 1 (2χ(M ) − K 2 ) + (W 2 ) 24 8

mod 2.

Since for a complex surface the Pontryagin class is: p1 (M ) = c21 − 2c2 , and the Hirzebruch signature theorem says σ(M ) = − 13 p1 (M )[M ], it follows that Theorem 4.7 is equivalent to:

IV.4. Spinc structures on complex surfaces

109

4.8 Theorem. Let M be as above, a compact complex manifold of dimension 2, and let W = 2D − K be a characteristic divisor of M . Then: σ(M ) − W 2 ≡ 8 dim H 0 (W ; D|W )

mod (16) .

This theorem is essentially an algebro-geometric variation of Rochlin’s signature theorem, the difference being that in Theorem 3.7, W has to be a smooth characteristic submanifold and the term on the right-hand side is the Arf invariant of a quadratic form on H1 (W ; Z2 ) defined topologically, while here W is a (possibly singular, non-reduced and reducible) characteristic divisor. Notice that W is defined by a holomorphic section of a line bundle L. Every such section can be approximated by a C∞ section st of L which is transversal to  of st is then a smooth, real submanifold of M repthe zero-section. The zero set W  is a characteristic submanifold resenting the same homology class as W . Hence W in the sense of Section 1, and 4.8 together with Rochlin’s theorem yield:  ) ≡ dim H 0 (W ; D|W ) Arf(W

mod (2) .

(4.9)

Is this a special case of a more general theorem? For instance, following [192] we  is canonically spin: see that the manifold W 4.10 Question. Is the mod (2) index (W ) = dim H 0 (W ; D|W ) equal to the mod (2)  ? And in higher dimensions? index of the Dirac operator on W For instance, if a real manifold M of dimension 8k + 4 has a spinc structure and W ⊂ M is a codimension 2 characteristic submanifold, then W is automatically spin. Each of these manifolds has a Dirac operator DM , DW ; the C ∞ version of Theorem 4.7 in higher dimensions (see [76]) would be to ask whether the index of DM reduced modulo 2 equals the mod (2) index of DW . I do not know the answer to these questions. 4.11 Remark. It is worth pointing out some special cases of Theorem 4.7: (i) If K < 0, then W = −K is characteristic with D = 0. We obtain, dim H 0 (−K, O) ≡ χ(M, O)

mod 2 .

(ii) If K > 0, then W = K = D is characteristic and one has: dim H 0 (K, KM |K ) ≡ χ(M, O)

mod 2 ,

because χ(M, KM ) = χ(M, O) (by [101]). (iii) If K is even, so that M is spin, then K 2 is divisible by 8 and: T d(M ) ≡

1 2 K 8

mod 2 ,

that is, T d(M ) is always an integer (by [101]), but it may or may not be an even integer, and this is given by the parity of 18 K 2 . Hence the Todd genus is even when K = 0 (c.f. [18, Cor. 2.ii]).

110

IV.5

Chapter IV. The General Index Theorem

A review of surface singularities

We want to use the previous results for compact manifolds to study germs of surface singularities. For this we need some preliminary background about resolutions of singularities. Since this important material is now standard in the literature, we only make a brief review, for completeness and to set up our notation and conventions. Consider first a singular (reduced) complex curve C in some smooth complex surface X. An important result for plane curves, due to Max Noether (1883), is that by a finite sequence of blowing ups we can always resolve the singularities of C. Let us explain this with a little more care (see [44] for a clear account of the subject). Given a smooth point x in a complex surface X, take local coordinates so that we identify the germ of X at x with that of C2 at 0. Let us take a small disc U around x and consider the map: γ : U − {x} → CP 1 which associates to each y ∈ U − {x} the point in CP 1 represented by the line determined by x and y. The graph of γ is an analytic subset of (U − {x}) × CP 1 , whose closure  = graph(γ) ⊂ (U − {x}) × CP 1 X  is obtained by removing turns out to be a smooth complex surface. Notice that X x from X and replacing it by the limits of lines converging to x. Thus we have  → X which is replaced x by a copy of CP 1 . There is a projection map π : X 1 −1 ∼ biholomorphic away from E = CP = π (x). This transformation is called the blow-up of X at x (in the literature this is called sometimes a σ-process or a monoidal transformation). Now, given the reduced (maybe reducible) singular curve C ⊂ X, with X a smooth complex surface, let x be a point in Csing , the singular set of C, and look  is called the  → X. The closure π −1 (C − x) in X at the blow-up of X at x, π : X  proper (or strict) transform of C under the blow-up, and denoted C. Notice that  is obtained by removing x from C and replacing it by the limits of lines which C  is analytic in X  and projects to C under are tangent to C − {x}. This curve C π; this curve may still be singular, but its singularities are simpler. We may now  blowing up X  at this point to repeat the process, choosing a singular point in C,   2 , which is the get π2 : X2 → X and then consider the proper transform of C in X −1 closure of (π2 ◦ π) (C − x), and so on. The theorem is (see [24, II.7.1] for a short proof): 5.1 Theorem. Let X be a smooth complex surface and C ⊂ X an embedded reduced curve. Then there is a smooth complex surface Y and a proper map τ : Y → X  of C obtained by a finite sequence of blow-ups, such that the proper transform C in Y is smooth. The curve E = τ −1 (C), is called the total transform of C in Y . It consists  and the divisor τ −1 (Csing ). The theorem above can be of the proper transform C refined to the following theorem, which will be used later in the text.

IV.5. A review of surface singularities

111

5.2 Theorem. Let X and C be as above. Then by performing finitely many blow-ups more, if necessary, we can assume that the whole total transform E = τ −1 (C) of C in Y has only ordinary normal double points as singularities, and these are all  of C. away from the proper transform C We recall that an ordinary normal double point is locally defined by the equation {xy = 0}. Now consider the germ (V, 0) of a normal complex surface singularity. The following important theorem has a long history. This was first stated by Jung but his proof was not complete, and was completed later by Hirzebruch. The first complete proof of Theorem 5.3 is due to O. Zariski; this was done using blowing ups and it was based on a previous proof by R. Walker (1935) which was not correct either. We refer to [24, Ch. III] for a proof. This result was later generalized by Hironaka to all dimensions and for (real or complex) analytic spaces with arbitrary singularities. 5.3 Theorem. Let (V, 0) be a normal complex surface singularity. For simplicity assume it is defined in a sufficiently small ball around the origin in some Cn , so that V ∗ = V − 0 is non-singular. Then there exists a non-singular complex surface V and a proper analytic map π : V → V, such that: (i) the inverse image of 0, E = π −1 (0), is a (connected, reduced) divisor in V , i.e., a union of 1-dimensional compact curves in V ; and (ii) the restriction of π to π −1 (V ∗ ) is a biholomorphic map between V −E and V ∗ . The surface V is called a resolution of the singularity of V , and π : V → V is the resolution map. Sometimes these are called desingularizations of the singularities instead of resolutions. The divisor E is called the exceptional divisor. Notice that “the” resolution of (V, 0) is not unique: given a resolution V we can obtain new resolutions by performing blow-ups at points in E. By Theorem 5.2 above, given a resolution, we can make blow-ups on it, if necessary, so that the divisor E in Theorem 5.3 is good, i.e.: (iii) each irreducible component Ei of E is non-singular; and (iv) E has normal crossings, i.e., Ei intersects Ej , i = j, in at most one point, where they meet transversally, and no three of them intersect. 5.4 Definition. A resolution π : V → V is good if its exceptional divisor is good, i.e., if it satisfies conditions (iii) and (iv) above. Some authors allow good resolutions to have irreducible components of E intersecting transversally in more than one point, and they reserve the name very good for resolutions as in Definition 5.4. This makes no big difference and we prefer to keep the notation of Definition 5.4. We recall that given a non-singular complex surface V and a Riemann surface S in it, the self-intersection of S, usually denoted by S · S or simply by S 2 , is the Euler class of its normal bundle ν(S) in V (which coincides with its Chern class)

112

Chapter IV. The General Index Theorem

evaluated in the fundamental cycle [S]. Equivalently, S · S is the number of zeroes, counted with signs, of a generic section of the normal bundle ν(S). It is an exercise to see that every time we make a blow-up on a smooth complex 2-manifold, we get a copy of CP 1 with self-intersection −1. It is remarkable that the converse is true: recall that a non-singular curve in a smooth complex surface is said to be exceptional of the first kind if it is a copy of CP 1 embedded with self-intersection −1, one has: ˜ be a non-singular complex surface 5.5 Theorem. (Castelnuovo’s criterium) Let X and C an exceptional curve of the first kind. Then S can be blown down analytically and we still get a non-singular surface X. This result is in fact a special case of a more general theorem of Castelnuovo for exceptional divisors of the first kind. We refer to [82, Ch. 3 ] for a proof. Notice that Definition 5.4 has the very important consequence of giving us a minimal model: π

5.6 Definition. A resolution X → V is minimal if given any other resolution π




p

X  → V , there is a proper analytic map X  → X such that π  = π ◦ p. One has (see [24, III.6.2]): 5.7 Theorem. Up to isomorphism, there exists a unique minimal resolution of V , and this is characterized by not containing non-singular rational curves with selfintersection −1. We remark that these statements are false in dimensions more than 2: there are no minimal resolutions in general. Notice that the minimal resolution may not be good. For instance (c.f. [57]), for the Brieskorn singularity z12 + z23 + z37 = 0 , the minimal resolution has an exceptional divisor consisting of three non-singular rational curves meeting at one point, so it is not good; making one blow-up at that point we obtain a good resolution, which has now a central curve which is a 2-sphere with self-intersection −1, and three other spheres, each meeting the central curve in one point and with self intersections −2, −3, −7. Something similar happens in general: we can make the minimal resolution good by performing blow-ups, if necessary, and there is a unique (up to isomorphism) minimal good resolution.  Consider now a divisor E = ri=1 Ei in a complex 2-manifold X, whose irreducible components Ei are non-singular, they all meet transversally and no three of them intersect. To such a divisor we can associate an r × r integral matrix A = ((Eij )), called the intersection matrix of E, as follows: on the diagonal ∆ of A we put the self-intersection numbers Ei2 ; and if a curve Ei meets Ej at Eij points, we put this number as the corresponding coefficient of A. So this is necessarily a

IV.5. A review of surface singularities

113

symmetric matrix, whose coefficients away from the diagonal ∆ are integers ≥ 0 and in ∆ we have the self-intersection numbers of the Ei , called the weights of these curves. We have the following remarkable theorems of Mumford and Grauert (see [24, III.2]): π

5.8 Theorem [176]. If E is the exceptional divisor of a resolution X → V , where V is a normal surface, then the intersection matrix A is negative definite (and the weights of the Ei are all negative numbers). 5.9 Theorem [91]. Conversely, if the divisor E in X is such that the intersection matrix A is negative definite, then we can blow down E analytically; we get a normal complex surface V , in general with a singularity at the image 0 of E, and the projection π : X → V is a good resolution of (V, 0) with exceptional divisor E. A divisor E as above is usually called an exceptional divisor, meaning by this that it can be blown down. It is said to be of the first kind when the blow-down is smooth. Notice that we can associate a weighted graph G = G(E) to a good exceptional divisor E in a complex 2-manifold X as follows: to each irreducible component Ei of E we associate a vertex vi , and if the curves Ei and Ej meet, then we join the vertices vi and vj by an edge. Each vertex has two integers attached to it: one is the genus gi ≥ 0 of the corresponding Riemann surface Ei ; the other is the weight wi = Ei2 ∈ Z, which is the self-intersection number of Ei in X. This weighted graph is called the dual graph of the exceptional divisor E, or the dual graph of the resolution when E is regarded as the exceptional set of a good resolution of a normal singularity. We observe that every finite graph Σ has associated a matrix I(Σ) called the matrix of adjacencies of the graph: it has zeroes in the diagonal and if a vertex vi is joined to vj by δij edges, then we put this number in the corresponding place of I(Σ). It follows that the intersection matrix of the exceptional divisor E is the result of taking the matrix of adjacencies of the dual graph and replacing its diagonal by the vector of weights w1 , . . . , wm , wi = Ei · Ei . A beautiful aspect of these constructions is that the dual graph of a resolution allows us to re-construct the topology of the resolution, and hence that of the link of the singularity. For this we need to introduce a construction known as plumbing. This was used already by Milnor to construct his first examples of exotic spheres and by Von Randow (1962) in relation with Seifert manifolds, though it was Hirzebruch who made this construction systematic. The plumbing construction is very nicely explained in [105] (see also [187, 75]), we just recall it here briefly. Let E be a real 2-dimensional oriented vector bundle over a Riemann surface S, and denote by D(E) its unit disc bundle for some metric. The total space of D(E), that we denote by the same symbol, is a 4-dimensional smooth manifold with boundary the unit sphere bundle S(E). Notice that, restricted to a small disc Dε in S, the manifold D(E) is a product of the form D2 × D2 , where the first disc

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Chapter IV. The General Index Theorem

is Dε ⊂ S and the second disc is in the fibres of E. Now suppose we are given two such bundles Ei , Ej , over Riemann surfaces Si , Sj . To perform plumbing on them we consider the total spaces of the corresponding unit disc bundles D(Ei ), D(Ej ), we choose small discs Di,ε , Dj,ε in Si , Sj , and take the restriction of D(Ei ), D(Ej ) to these discs. Each of them is of the form D2 × D2 as above. We now identify each point (x, y) ∈ Di,ε × D2 ⊂ D(Ei ) with the corresponding point (y, x) ∈ Dj,ε × D2 ⊂ D(Ej ), i.e., interchanging base points in one of them with fibre points in the other. The result is a 4-dimensional, oriented manifold with boundary and with corners, which can be smoothed off in a unique way up to isotopy (see [60]). We denote this manifold by P (Ei , Ej ). One says that P (Ei , Ej ) is obtained by plumbing the bundles Ei and Ej over the Riemann surfaces Si and Sj .

Figure 9: Plumbing line bundles over circles.

The boundary S(Ei , Ej ) = ∂P (Ei , Ej ) of this 4-manifold is obtained by plumbing the corresponding sphere bundles Si (E) and Sj (E): we remove from Si (E) and Sj (E) the interior of the solid tori ∂Di × D2 and similarly for Ej . Thus we get two 3-manifolds with boundary a torus S1 × S1 in each; we then identify these boundaries by gluing the meridians in one torus to the parallels in the other. The result is a 3-manifold with corners, which can be smoothed off in a unique way up to isotopy. The surfaces Si , Sj are naturally embedded in P (Ei , Ej ) as the zero-sections of the corresponding bundles, and they meet transversally in one point. Notice that the manifolds one gets in this way are entirely described, up to diffeomorphism, by the genera of the Riemann surfaces Si , Sj , and by the Euler classes of the corresponding bundles, since these classes determine the isomorphism class of the bundles.

IV.5. A review of surface singularities

115

5.10 Definition A plumbing graph is a triple (Σ, w, g) consisting of a finite graph Σ with vertices v1 , . . . , vr , r ≥ 1 and with no loops, a vector w of weights, w = (w1 , . . . , wr ), wi ∈ Z, and a vector g = (g1 , . . . , gr ) of genera, gi ∈ N. So the dual graph of a good resolution of a normal singularity is a plumbing graph with negative definite intersection matrix. In this definition, by a loop we mean an arrow that begins and ends at the same vertex, and we do not allow this (geometrically this means a singular curve in the exceptional divisor that has a double crossing). There can be cycles, i.e., a chain of vertices and edges that returns to itself after a certain time. In [184] Neumann considers a slightly more general situation of plumbing graphs than the one envisaged here, which is important for the plumbing calculus developed there, but it does not really make a difference for the present work. Now, given a plumbing graph we may perform plumbing according to the graph: for each vertex vi take a Riemann surface Si of genus gi and an oriented 2-plane bundle Ei over Si with Euler class wi . If there is an edge between the vertices vi and vj , we plumb the corresponding bundles as above. If a vertex vi is joined with other vertices, we choose pairwise disjoint small discs in each surface, as many as one has adjacent vertices, and perform plumbing by pairs as above. The result is a 4-dimensional manifold P(E) with boundary S(E).  It follows from the construction that the manifold P(E) contains the union E = Si as a deformation retract, and these surfaces are contained in P(E) with self-intersection wi . Hence the homology of P(E) is that of E, and the intersection form on P(E) is given by the intersection matrix of its corresponding graph. A manifold obtained in this way is known as a plumbed manifold, and this term may refer either to the 4-manifold P(E) with boundary, or to its boundary, which is a 3-manifold. Notice that if the plumbing graph (Σ, w, g) is the dual graph of a resolution π : V → V , then the manifold P(E) is diffeomorphic to a regular neighborhood of the exceptional set E in the resolution, which may be taken to be of the form π −1 (V ∩ Dε ), where Dε is a small closed disc in the ambient space Cn with centre at 0. Since the resolution map is a biholomorphism away from E, it follows that the boundary S(E) is diffeomorphic to the link of V , a fact that we state as a theorem: 5.11 Theorem. Let π : V → V be a good resolution of (V, 0), a normal surface singularity. Then the irreducible components E1 , . . . , Er of the exceptional divisor E determine a plumbing graph G(E), called the dual graph of the resolution, and performing plumbing according to this graph we obtain a 4-manifold homeomorphic to π −1 (V ∩ Dε ) ⊂ V , whose boundary is the link M , where Dε is a small closed disc in the ambient space Cn with centre at 0. Hence Theorems 5.8 and 5.9 say that a plumbing graph is the dual graph of a resolution if and only if its intersection matrix is negative definite. This gives a necessary and sufficient condition for an oriented 3-manifold to be the link of a surface singularity. A natural question is:

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Chapter IV. The General Index Theorem

5.12 Question. Which plumbed 3-manifolds with negative-definite intersection matrix arise as links of isolated singularities of hypersurfaces in C3 ? This is for me one of the most beautiful open questions in singularities. It is clear that if a 3-manifold M can be obtained in this way, then M can be embedded in S5 with trivial normal bundle. One may think that this could be a first step towards answering Question 5.12. However every orientable, closed 3manifold embeds in S5 with trivial normal bundle, so this is not an obstruction; hence the problem is not only topological. This is actually a very subtle problem and a number of people have worked on it obtaining interesting partial results (see for instance [265]). 5.13 Remark. It is worth noticing that the condition on a plumbing graph to be the dual graph of a resolution depends only on the graph and its weights, but not on the genera assigned to the vertices. I do not know if there is any interesting relationship between singularities with the same weighted graph but different genera. We also remark that given any finite graph, for almost every set of negative weights that we assign to the vertices, the corresponding intersection matrix is negative definite (see [119]) and therefore is the dual graph of a surface singularity.

IV.6

Gorenstein and numerically Gorenstein singularities

The concept of numerically Gorenstein singularities was introduced by A. Durfee in [64]; these could as well be called “topologically Gorenstein” surface singularities, by Corollary 6.3 below. 6.1 Definition. A normal surface singularity germ (V, 0) is numerically Gorenstein if the complex tangent bundle T V ∗ of V ∗ = V − 0 is C∞ trivial. It is worth noting that this definition is less innocent than it looks. Obviously Definition 6.1 is equivalent to demanding that T V ∗ |M be trivial. As a real vector bundle T V ∗ |M splits as the direct sum T M ⊕ ν(M ), where M is the link of V , T M its tangent bundle and ν(M ) its normal bundle for some metric. The bundle T M is trivial, because every oriented 3-manifold is parallelizable; and ν(M ) is also trivial, because a hypersurface in an oriented manifold is orientable iff it has trivial normal bundle, and we know that M is orientable. Hence T V ∗ is always trivial as a real vector bundle. Thus Definition 6.1 does ask for T V ∗ to be trivial as a complex bundle; but it does not ask for T V ∗ to be holomorphically trivial: the ZariskiLipman conjecture (still open) claims that this happens iff V is smooth at 0 (this conjecture is a theorem in many cases, as for instance for hypersurface germs). The following lemma was noticed in [64, 1.1] and summarizes well-known properties of vector bundles.

IV.6. Gorenstein and numerically Gorenstein

117

6.2 Lemma. Let ξ be a 2-dimensional complex bundle over a connected, open C ∞ manifold X. The following conditions are equivalent: (i) ξ is trivial; (ii) ξ is stably trivial; (iii) the first Chern class of ξ vanishes; 2 (iv) the second exterior power ξ is a trivial bundle. The proof is easy: we recall that a bundle is stably trivial if its tangent bundle plus a trivial bundle is trivial. For instance the tangent bundle of the 2-sphere is not trivial, but when we add to it the normal bundle of S2 in R3 we get T R3 |S2 , which is trivial. Hence T S2 is stably trivial. So (i) implies (ii) by definition. But also (ii) implies (i), because it is easy to see that, in general, if ζ is a complex n-plane bundle over a manifold N of dimension 2n and ζ is stably trivial, then there is a unique obstruction to have it actually trivial, and this is a cohomology class in top dimension 2n (see [113]). Since in our case we are assuming X is open and connected, then one has H 4 (M ) = 0 and therefore (ii) implies (i). That (i) implies (iii) is by definition of the Chern classes. Now, for every complex vector bundle one has that its to sign, the Chern class of its  first Chern class is, up  second exterior power 2 ξ. Hence (iii) implies c1 ( 2 ξ) = 0 and this implies 2 ξ is trivial, because a line bundle is trivial iff its Chern class vanishes; so (iii) and (iv) are equivalent. Finally, since H 4 (M ) = 0, the construction we gave above of the Chern classes via obstruction theory shows that one can always construct a never-zero section s1 of ξ on all of X. Now, c1 (ξ) = 0 is equivalent to saying that we can construct a second section s2 of ξ which is never-zero and linearly independent of s1 on the 2-skeleton of X (for some triangulation). Then we can extend this section s2 to all of X using the stepwise process of Section 1, since
πi (S1 ) vanishes for i > 1. 6.3 Corollary. The following conditions are equivalent: (i) the germ (V, 0) is numerically Gorenstein; 2 (ii) the bundle T V ∗ is topologically trivial; (iii) the structure group of the bundle T V ∗ can be reduced to SU (2); (iv) if π : V → V is any resolution of V , then the restriction of the tangent bundle
= π −1 (M ) , where M = V ∩ S2n−1 is the link, is a trivial bundle; T V to M ε

(v) the Chern class c1 (T V ) can be represented by a relative cohomology class in
; Z), whose Lefschetz dual −K ∈ H2 (V ; Z) is uniquely characterized H 2 (V , M by the adjunction formula: for every compact, non-singular curve C in V one has: (6.4) 2gC − 2 = C 2 + K · C. Proof. The equivalence (i) ⇔ (ii) is a special case of the equivalence (i) ⇔ (iv) in Lemma 6.2. The equivalence (ii) ⇔ (iii) was proved in III.6. Now, it is clear that (i) implies (iv) since π is a biholomorphism away from E = π −1 (0); conversely,

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Chapter IV. The General Index Theorem

it is clear that (iv) implies that T V ∗ |M is trivial, hence T V ∗ is trivial by the conical structure of V . Also, if T V |M
is trivial, then a choice of a trivialization T of this bundle determines representatives of the Chern classes of T V that vanishes
) and it is determined by over M ; the top relative class c2 (V ; T ) lives in H 4 (V , M the degree of T , i.e., by the number of zeroes (counted with multiplicities) of an extension to V of one of the sections that determine T . The class c2 (V ; T ) lives
), as stated in (v) above, and it must satisfy the adjunction formula in H 2 (V , M because for every C as above one has a splitting: T V |
∼ = T C ⊕ ν(C) M

as C ∞ bundles, where ν is the normal bundle. Thus one has c1 (T V |M
)[C] = c1 (T C)[C] + c1 (ν(C))[C] , where [C] is the fundamental cycle. Hence, −K · C = (2 − 2gc ) + C 2 as claimed. Notice that these equations characterize uniquely the class K, since the intersection matrix is negative definite. Thus (iv) ⇒ (v). Finally, (v) implies
× (−ε, ε) that the first Chern class of the bundle T V |M×(−ε,ε) vanishes, where M

in V . Hence (v) and Lemma 6.2 imply (iv).
is a tubular neighbourhood of M Notice that the manifold V has the exceptional set E = π −1 (0) as a deformation retract. Hence, if the resolution is good, then the irreducible components E1 , . . . , Er of E are non-singular and they form a basis for the homology of V . Thus, in order to characterize uniquely the canonical class K, it is enough to consider these curves in the adjunction formula (6.4). 6.5 Definition. The class K is the canonical class of the resolution. 6.6 Corollary. Every Gorenstein singularity 2is ∗numerically Gorenstein. More precisely, Gorenstein means that the bundle T (V − 0) is holomorphically trivial, while numerically Gorenstein means that this bundle is topologically trivial. Notice that the canonical K corresponds to the zero-set of a differen2 class T ∗ (V − 0). If V is assumed to be Gorenstein, then tiable section of the bundle K can be taken to be a divisor, i.e., defined by a meromorphic 2-form. We also remark that regardless of whether or not (V, 0) is Gorenstein, the intersection matrix is negative definite and therefore the adjunction formula determines a unique canonical class, which may have rational coefficients (see [64, 119]). The geometric reason behind this is that the class determined by the adjunction formula is, by construction, concentrated in the exceptional divisor, it is always a rational linear combination of the form: K = n1 E1 + · · · + nr Er ; however, when the singularity is not numerically Gorenstein, one cannot possibly have a representative of the first Chern class of T V that is localized on E.

IV.6. Gorenstein and numerically Gorenstein

119

6.7 Examples. (i) First consider a plumbing graph with only one vertex: •

(w, g) where w ∈ Z is the weight and g ∈ N is the genus. The intersection matrix is now 1 × 1, so it is negative definite iff w < 0; hence all of these are graphs of normal surface singularities. By the adjunction formula, the canonical class is   2g − 2 −1 · E , K= w where E is the curve of genus g represented by the vertex. Hence the corresponding singularities are numerically Gorenstein iff 2g − 2 ≡ 0 mod w. (ii) If the graph Σ is one of the classical Dynkin diagrams An , Dn , E6 , E7 or E8 , with all weights equal to −2 and all genera 0, then the intersection matrix is negative definite, the corresponding singularities are the Klein singularities studied in Chapter II, i.e., those of the form Γ\C2 for a finite subgroup of SU (2), and a trivial computation shows that K ≡ 0, see for instance [65]. (iii) If the graph is a star, with a central curve E0 of genus g ≥ 0, weight w = 2−2g −n, and all other vertices r represent spheres Ei with weights {−α1 , . . . , −αr } satisfying αi > 1 and w < i=1 −1 αi , then the intersection matrix is negative definite and the corresponding plumbing graph represents the minimal good resolution of a Dolgachev singularity [57], i.e., one of the singularities considered in III.3.3, which corresponds to taking a cocompact Fuchsian group Γ ⊂ P SL(2, R), making it act on T H = H × C and then blown down the zero-section. By the adjunction formula, the canonical class is now K = −2E0 −

r 

Ei .

i=1

(iv) Now start with a discrete subgroup Γ ⊂ SL(2, R) with compact quotient Γ\SL(2, R) which does not contain the centre ±Id, and construct a surface singularity as in III.3.3, i.e., we make Γ act on H × C2 , where C2 denotes the 2-fold cyclic cover of C branched at 0. The quotient contains (Γ\H × {0}) as a divisor, which can be blown down to get a normal singularity. The resolution graph is now of the form (see [194] or [231, p. 351]): where {g; α1 , . . . , αr } is the signature of the Fuchsian group obtained by mapping Γ to P SL(2, R). All the αi are (automatically) odd numbers and all vertices represent spheres, except the centre E0 which has genus g ≥ 0. In this case one has that the canonical class is r r   Ei,1 − Ei,2 . K = −3E0 − 2 i=1

i=1

120

Chapter IV. The General Index Theorem

− 12 (α3 + 1)

− 12 (α2 + 1) −2 −2 − 12 (αn + 1) −2

(g; 1 − g − n)

−2

− 12 (α1 + 1)

Figure 10: Resolution graph for SL(2, R)-singularities.

v) More generally, we recall from Chapter III that if (V, 0) is a normal singularity with a good C∗ -action, then its link is a Seifert manifold of the form Γ\G where
G is either SU (2), the Heisenberg group N or SL(2, R), the universal cover of
P SL(2, R), and Γ is a uniform subgroup of G. Consider the case Γ ⊂ SL(2, R), which is the most interesting. The minimal good resolution of these singularities is given in [194]. It is always a star with a central curve E0 of some genus g ≥ 0, as many branches as exceptional fibres of the corresponding Seifert fibration and all other vertices represent spheres; the lengths of the branches and their weights are all determined by the corresponding Seifert invariants. The canonical class was essentially computed in [231]. The remarkable thing is ([231, 2.4], c.f. III.2): 6.8 Proposition. Let −mo be the coefficient of the canonical class K of the minimal
resolution  of an  SL(2, R)-singularity. Let r = m0 − 1. Then the group Γ has index r in p−1 p(Γ) . Or equivalently, if we denote by Γr the projection of Γ to the r-fold cyclic cover P SL(2, R)r of P SL(2, R), then:
R); and (i) Γr \P SL(2, R)r is diffeomorphic to Γ\SL(2, (ii) Γr is isomorphic to its image in P SL(2, R). 6.9 Remark. We pointed out earlier that the property for a plumbing graph to have negative definite intersection matrix depends only on the weights and not on the genera assigned to the vertices. However the property of being the dual graph of a numerically Gorenstein singularity does depend on the genera too, since these do appear in the adjunction formula. One can prove (see [119, 3.10]) that given any graph Γ and weights w = (w1 , . . . , wr ) so that the intersection matrix E is negative definite, there are infinitely many different possible genera for the vertices that make it correspond to a numerically Gorenstein singularity. In fact one can say more in some cases. For example, we know that K represents the 1st Chern class of the resolution V . Hence K is an even class iff V admits a spin structure.

IV.7. Laufer’s formula

121

One can prove that if the weights are even, then there are infinitely many different genera for which the class K is even. Furthermore, if the weights are even and the determinant det(E) of E is odd, then K is integral iff it is even. As a consequence, if a complete intersection surface singularity has a resolution with even weights and det(E) is odd, then K is even.

IV.7

An application of Riemann-Roch: Laufer’s formula

There is an important invariant of surface singularities called the geometric genus, which extends to surface singularities the genus of Riemann surfaces (regarded as the number of linearly independent differential forms). M. Artin introduced it in [1] to define the concept of rational singularities, which means geometric genus 0. This has been much used and studied by several authors for various purposes, as for instance F. Hirzebruch, H.B. Laufer, K. Saito, S.T. Yau and many more. To define the geometric genus ρg = ρg (V, 0) of a normal surface singularity (V, 0), let π : V → V be a resolution of V . Then ρg (V, 0) is: ρg (V, 0) = dim R1 π ∗ OV the dimension of the direct image sheaf R1 π ∗ OV , which is a coherent sheaf concentrated at 0. It was noticed in [14] that this number is independent of the choice of resolution (since it is invariant under blowing ups) and it can also be computed as: ρg = dim H 1 (X, OX ) , where the term on the right must be understood as an inverse limit over a fundamental set of open neighborhoods of the exceptional divisor E = π −1 (0). The formula of Laufer in [123] (and later generalized by others, as we explain below) gives a very interesting relationship between the geometric genus and invariants of the singularity which are associated to deformations of it, namely the Milnor number in the case of complete intersection germs, or an extension of it for more general normal singularities which are Gorenstein and smoothable (see below). Let us look first at the classical case of a hypersurface germ in C3 , as considered by Laufer. We notice that every such germ is automatically Gorenstein (see for instance III.3.2 above), so given a resolution π : V → V , then a neverzero holomorphic 2-form on V − 0 lifts by π to a never-zero holomorphic 2-form on V − E, which extends to a meromorphic form on V , possibly with poles and zeroes in the divisor E. Thus we know that the canonical bundle KV is holomorphically trivial away from E and we can choose the canonical divisor K to be vertical, i.e., with support in E. If we assume that the resolution V is good, then the irreducible components E1 , · · · , Er of E are non-singular curves and K will necessarily be an integral linear combination of these (see the previous section).

122

Chapter IV. The General Index Theorem

Let f : (U ⊂ C3 , ) → (C, 0) be a function-germ that defines V . Then one has the Milnor number µ of f defined in I.7. We know (see for instance [120, 7.1]) that up to a local change of coordinates, we can assume f is a polynomial map. Compactify C3 to CP 3 , denote by V¯ the closure of V in CP 3 and let V¯t be the compactification of Vt = f −1 (t) for some t = 0, |t| small. As noted by Laufer, we can assume that V¯ is non-singular away from 0 and V¯t is non-singular. One obviously has χ(V¯t ) = χ(V¯ ) + µ, where µ = χ(Vt ) − 1 is the Milnor number. Now, given a resolution π : V → V of V , let V ∗ be the resolution of V¯ that has V as open set. Topologically this means that V , V and Vt have all been compactified by adding “the same divisor” K∞ at ∞. Letting E = π −1 (0), one has: χ(V ∗ ) = χ(V¯ ) + χ(E) − 1, since V ∗ is obtained by removing 0 from V¯ and replacing it by E. Also, the Riemann-Roch formula of Section 2 says that the analytic Euler characteristic is: χ(V ∗ , O) =

1 1 (K 2 + χ(V ∗ )) = (K 2 + KV2 + χ(V ∗ )), 12 V ∗ 12 ∞

and

1 (K 2 + χ(V¯t )), 12 ∞ because KV2t = 0 since the canonical bundle of Vt is trivial. Finally, using Mayer-Vietoris ([7, p. 236]) for the cohomology of V ∗ = V ∪ V ∗ − E and V¯ = V ∪ V¯ − 0 one gets: χ(V¯t , O) =

χ(V ∗ , O) = χ(V¯ , O) − h1 (V , O). Putting these equations together Laufer[123] arrives at his formula: µ + 1 = χ(V ) + K 2 + 12ρg ,

(7.1)

where χ(V ) is the topological Euler-Poincar´e characteristic of the resolution and K 2 is the self-intersection number of the canonical divisor. This result of Laufer shows also that the geometric genus cannot in general be defined by the topology of V alone (c.f. [181, 182]). For instance, the Brieskorn singularities with (p, q, r) being (3, 5, 15) and (2, 9, 18) are homeomorphic but their Milnor numbers are 112 and 136, respectively, so their geometric genera are different. Laufer’s proof extends with minor changes to complete intersection germs, and he conjectured that the same formula should hold for normal Gorenstein smoothable singularities, just replacing the left-hand side by the Euler-Poincar´e characteristic of the smoothing. Let us explain this.

IV.7. Laufer’s formula

123

Given a normal surface singularity (V, 0), a smoothing of V means a flat deformation of V where all nearby fibres are smooth. To be precise: 7.2 Definition. An isolated complex singularity germ (X, P ) of dimension n ≥ 1 is smoothable if there exists a complex analytic space (W, 0) of dimension n + 1 and a proper analytic map: F : W −→ D ⊂ C , where D is an open disc with centre at 0, such that: (i) it is flat, i.e., it is not a zero divisor in the local ring of W at 0; (ii) F −1 (0) is isomorphic to V ; and (iii) F −1 (t) is non-singular for t = 0. It follows that W has (at most) an isolated singularity at 0, which is normal when X is normal at P , and one has (by Theorem I.4.3) a fibre bundle F |F −1 (D−0) : F −1 (D − 0) −→ D − 0 with non-singular fibres Xt = F −1 (t). For simplicity we call smoothings of X to both, the map F and the fibres Xt . If W is smooth, then its germ at 0 is essentially that of Cn+1 at 0. The picture one has in general is similar to the one of hypersurfaces and their corresponding Milnor fibration, but now the ambient space W can be itself singular at 0. For hypersurfaces and complete intersection germs the smoothing is unique up to isomorphism (because the base of the versal deformation space is connected) and it is given by the Milnor fibration. However there are normal singularities which are not smoothable (e.g., some triangle singularities of Dolgachev, see for instance [199, 147]), and there are also surface singularities which are smoothable but they have different smoothings whose fibres have distinct Euler-Poincar´e characteristics (see [199]). Laufer’s formula explained above was later extended by Steenbrink [241] to surface singularities which are Gorenstein and smoothable. His proof is via mixed Hodge structures, but it can be proved exactly with the same method of Laufer, using a result in [225] and a globalization theorem of Looijenga, conjectured by J. Wahl in [258]. 7.3 Theorem. (E. Looijenga [148]) Let F : (W, x) → (C, 0) be a smoothing of an isolated singularity (X, x). Then there is, up to isomorphism, a global projective compactification of the smoothing. That is, there exists a flat projective morphism F : Z → C, a point z ∈ Z0 = F −1 (0) and an isomorphism h : (W, x) → (Z, z) such that F = F ◦ h and F is smooth along Z0 − z. We refer to the appendix in [148] for the proof of this theorem. As pointed out by Wahl in [258, p. 220], this assertion allows us to use the Riemann-Roch theorem to compare the cohomology of the singular variety, the smoothing and the resolution, just as in Laufer’s proof. Everything works in exactly the same

124

Chapter IV. The General Index Theorem

way as for hypersurfaces, except for one more point that has to be proved: for hypersurfaces (or complete intersections) of any dimension, the canonical bundle of the smoothing (the Milnor fibre) is holomorphically trivial. To see this one may, for instance, take the form dz1 ∧ · · · ∧ dzn+k in the ambient space, and contract it using the gradient vector fields of the function germs that define the complete intersection singularity. The same statement holds for smoothings of isolated, normal singularities in all dimensions. 7.4 Proposition [225]. Let F : (W, 0) → C be a smoothing of a normal, Gorenstein singularity (V, 0) of dimension n ≥ 1, and letTF denote the tangent bundle along n ∗ TF is holomorphically trivial. the fibres of F on W − V . Then the bundle In the following section we will see that Proposition 7.4, which is obvious for the algebraic-geometers, has an important topological implication in the case of surfaces, and this proved a conjecture of Durfee in [64]. Proof. Consider the smoothing F : W −→ D ⊂ C , so each fibre Vt = F −1 (t), t = 0, is non-singular. Since V is Gorenstein at 0 and F is flat, it follows that W is also Gorenstein at 0 (see for instance [97, V.9.6]). Hence there exists a nowherevanishing holomorphic (n + 1)-form ω  on W − 0. Since F is flat, we have an exact sequence of vector bundles over W − 0: 0 → TF → T (W − 0) → F ∗ (T D) → 0 , where TF denotes the tangent bundle along the fibres of F . Hence, n+1

T ∗ (W − 0) ∼ =

n

TF∗ ⊗ F ∗ (T ∗ D)

and the above holomorphic (n + 1)-form on W − 0 defines a nowhere-vanishing holomorphic n-form n ∗ on each non-singular fibre of the smoothing and trivializes the TF .
whole bundle The fact that W is Gorenstein at 0 is fairly standard in algebraic geometry and can be proved as follows: the local ring of V at 0 is:    OV,0 ∼ = OW,0 (F ) by hypothesis. Since V is Gorenstein, it is Cohen-Macaulay, so there are elements g¯1 , . . . , g¯n ∈ OV,0 which form a regular  sequence, i.e., g¯1 is not a zero-divisor in OV,0 , g¯2 is not a zero-divisor in OV,0 (¯ g1 ) and so on, and such that  g1 , g¯2 , . . . , g¯n )} ∼ HomOV,0 {C , OV,0 (¯ = C. Hence if g1 , . . . , gn ∈ OW,p are extensions of g¯1 , . . . , g¯n (which exist by normality), then (F , g1 , . . . , gn ) form a regular sequence in OW,p and 
HomOW,0 {C , OW,0 (F , g1 , . . . , gn )} ∼ = C.

IV.7. Laufer’s formula

125

With this, one obtains the Laufer-Steenbrink formula in general: 7.5 Theorem. Let (V, 0) be a 2-dimensional normal singularity which is Gorenstein and smoothable. Let Vt be a smoothing of V and let V → V be a resolution. Then one has: χ(Vt ) = χ(V ) + K 2 + 12ρg (V, 0) , where χ(·) is the usual (topological) Euler-Poincar´e characteristic, K is the canonical class of V and ρg (V, 0) = dim H 1 (V , OV ) is the geometric genus of 0 ∈ V . 7.6 Remarks. (i) It is obvious that χ(V ) depends on the choice of resolution, as can be seen just by noticing that this increases by 1 when we make a blow-up. But it is an exercise to show that the sum χ(V ) + K 2 does not depend on the choice of resolution, and we know that neither does ρg (V, 0). Hence Theorem 7.3 implies that for normal surface, smoothable singularities, being Gorenstein implies that at least χ(Vt ) does not depend on the choice of the smoothing (which is not the case for surface singularities in general, by [199]). Notice that the right-hand side of the formula in Theorem 7.5 makes sense even for non-smoothable singularities, and it depends only on the analytic structure of V , not on the choice of resolution. It is thus natural to ask what plays the role of χ(Vt ) when there is not a smoothing? In other words, what should we put in the left-hand side of Theorem 7.5 when there is no smoothing? This question was asked of me by Dolgachev and I do not know the answer. Presumably this will be in terms of the Seiberg-Witten invariant of the link (see §11 below). (ii) There is an interesting generalization in [148] of the above formula to higher dimensions, also based on the generalized Riemann-Roch theorem. To state this, the first observation that Looijenga makes is that using a deep theorem of O. Gabber (or also by work of Steenbrink) one has that given an isolated singularity (V, 0) of dimension n ≥ 1, the subalgebra of H ∗ (V − 0) generated by the Chern classes of the tangent bundle T (V − 0) is trivial in dimensions ≥ n. For complete intersection germs this is an obvious fact, since the bundle T (V − 0) is itself trivial, but for singularities in general it is not so obvious. Now let X denote either a resolution of the singularity or a smoothing of it (if it is smoothable), and think of it as being a compact manifold with boundary the link M = ∂X. Given any polynomial P (c1 , . . . , cn ) in the Chern classes of X of top degree n, the previous observation tells us that we can lift the corresponding cohomology class to P ∈ H ∗ (X, ∂X). Elementary properties of the cup product of relative cohomology classes yield that if the polynomial P is decomposable (i.e., if it is the product of at least two Chern classes of degree less than n), then the lifting P does not depend on the choice of lifting, so it is canonically defined. In the case of the top class cn this is not so, and Looijenga shows that one can always do it in such a way that the class one gets is the Lefschetz dual of χ(X) in H 2n (X, ∂X). With this one may now define the Todd genus of X, just as in Section 2 above, by taking the nth Todd polynomial in these classes, and evaluating it on the fundamental

126

Chapter IV. The General Index Theorem

cycle of X. Taking the representative of (V, 0) small enough, we can assume that the smoothing Vt is Stein and therefore (by the theorem of Andreotti-Frankel) all its cohomology groups vanish in dimensions > n. Thus T d(Vt ) is determined only by χ(Vt ) and, if n = 2m is even, by c2m . Now denote by π : V → V a resolution of V . Then Looijenga’s theorem says:   (−1)i−1 hi (V ; OV ) − dim π∗ OV OV . T dn (Vt ) = T dn (V ) + i>0

IV.8

Geometric genus, spinc structures and characteristic divisors

This section is based on [76], where Riemann-Roch is used to explore the relationship between the geometric genus of normal, Gorenstein surface singularities and the dimension of the space of sections of holomorphic line bundles associated with spinc -structures on a resolution of the singularity. Essentially everything we say here holds in complex dimensions 4k + 2, with some obvious modifications; we restrict to k = 0 for simplicity, and we refer to [76] for details. We know that since V is Gorenstein, given any resolution π : V → V , we may consider the canonical divisor K to have support in the exceptional divisor π −1 (0). And we know from §4 that on a complex manifold, the anti-canonical divisor determines a canonical spinc -structure. In the sequel we shall consider divisors in V of the form W = 2D − KV with D vertical, i.e., with π(|D|) = 0. The bundle L of such a divisor determines a spinc structure on V for which L is the determinant bundle. We call these characteristic divisors of the resolution. Given a characteristic divisor W = 2D − KV and its bundle L, we define the invariant:  0 , if W = 0 (W ) = dim H 0 (W, D|W ) , if W = 0; The reduction of (W ) modulo 2 is the invariant introduced in §4. One has ([76, 3.5 & 4.1]): 8.1 Theorem. Let (V, 0) be as above, a normal Gorenstein surface singularity, and let ρg (V, 0) be its geometric genus. Let π : V → V be a resolution of V and KV its canonical divisor, chosen so that it is vertical. For every characteristic divisor W = 2D − K with D vertical, one has: (i) The integer (W 2 − KV2 ) is divisible by 8, and (W ) +

1 (W 2 − KV2 ) ≡ ρg (V, 0) 8

mod (2) .

(ii) Moreover, if D = 0 or D > 0 and −D is relatively ample for π, then: ρg (V, 0) = (W ) +

1 (W 2 − KV2 ) . 8

IV.8. Geometric genus and characteristic divisors

127

(iii) If KV ≤ 0, then for all D ≥ 0, setting W = 2D − KV , one has: ρg (V, 0) = (W ) +

1 (W 2 − KV2 ) . 8

The proof of this theorem in [76, §3,S4] is based on the Hirzebruch-RiemannRoch theorem and on several vanishing theorems. The idea is to compactify the resolution of the singularity, then use Riemann-Roch and cancel down the terms that come from contributions away from the exceptional divisor. More precisely, given any resolution π : V → V , a smooth compactification V ∗ of V and a vertical divisor D, for every a ∈ Z we can define an invariant g(a · D) ∈ Z as the difference of the analytic Euler characteristic     g(a · D) = χ OV ∗ (a · D) − χ OV ∗ . It is an exercise to verify that the additivity of Euler characteristics for short exact sequences and the Riemann-Roch theorem imply: 8.2 Properties of g(a · D): (i) it is independent of the compactification V ∗ chosen; (ii) if KV is the canonical divisor of V , then g(a · D) = 12 (a · D) · (a · D − KV ); (iii) for W = 2D − KV one has: g(D) = 18 (W 2 − KV2 ) . Having this one can prove that for every effective vertical divisor ∆: (W ) + g(D) ≡ (W + 2∆) + g(D + ∆)

mod (2) .

(8.3)

Thus statement (ii) in Theorem 8.1 implies statement (i), since we can always choose some effective vertical divisor ∆ such that D + ∆ > 0 and −(D + ∆) is relatively ample for π. The proof of statements (ii) and (iii) in Theorem 8.1 does rely on several vanishing theorems given in [76] and we refer to that article. Since, by [176], for the minimal resolution of a surface singularity it is always the case that the canonical divisor is ≤ 0, one has: 8.4 Corollary. If the resolution V is minimal, then ρg (V, 0) = dim H 0 (−KV , O) = dim H 0 (W, D|W ) +

1 (W 2 − KV2 ) 8

for all W = 2D − KV with D ≥ 0. 8.5 Remarks. (i) If the canonical divisor KV of some resolution is even, one has that the resolution must be minimal (by [119]) and admits a spin structure; in this case one has (essentially from Theorem 8.1) that KV2 is divisible by 8 and 1 2 K ≡ ρg (V, 0) 8 V

mod (2) .

128

Chapter IV. The General Index Theorem

 If KV is not even, the most natural choice of a characteristic divisor is W0 = Ei , where the sum is taken over all components of the exceptional divisor E = π −1 (0) which have odd multiplicity in KV . With this choice, assuming that the resolution is good and the Betti number b1 (E) = dim H 1 (E; Z) is 0, one gets: 1 (W 2 − KV2 ) ≡ ρg (V, 0) 8 0

mod (2)

since b1 (E) = 0 implies that H 0 (E, ωE ) = 0 and hence H 0 (W0 , ωW0 ) = 0, where ω∗ is the canonical sheaf (c.f. §4 above). Hence H 0 (W0 , OW0 (D)) = 0. This remark, together with the combinatorial argument contained in [140], imply that Theorem 2 in [140] holds without the assumption that the singularity is smoothable, i.e., if the determinant of the intersection matrix of E is odd, then one always has: KV2 + 8ρg (V, 0) ≡ W02

mod (16) .

(ii) In [140] the authors present four examples of singularities with KV even and for which the relation 18 KV2 ≡ ρg (V, 0) mod (2) does not hold. Alas their computations were mistaken (c.f. [141]), as proven by Theorem 8.1.

IV.9

On the signature of smoothings of surface singularities

In Section 7 we explained Laufer’s formula expressing the Euler-Poincar´e characteristic of a smoothing of a normal Gorenstein smoothable singularity in terms of invariants associated to a resolution of it. Section 8 gives relations between the geometric genus and the spaces of sections of characteristic line bundles over a resolution, particularly with the canonical bundle. All these formulae are proved by using the Hirzebruch-Riemann-Roch theorem, applied to projective compactifications of V (and in Section 7 of a “Milnor fibre” Vt = f −1 (t), where f is the smoothing), and then cancelling down terms that come from the contributions away from the origin. In this section we are concerned with a formula obtained by A. Durfee in [64], which is in the same spirit as Laufer’s but uses the signature defect, introduced by Hirzebruch, instead of the Hirzebruch-Riemann-Roch formula. The result is an interesting and useful formula for the signature of smoothings of Gorenstein singularities. One may notice that, since for compact complex surfaces the Todd genus and the signature are both expressible in terms of the canonical class and the Euler-Poincar´e characteristic, the formulas of Durfee and Laufer turn out to be essentially equivalent. Let us define first the signature defect of a framed 3-manifold. This invariant of 3-manifolds was defined by Hirzebruch via his signature theorem, and it was later generalized by Morita and (notably) by Atiyah, Patodi and Singer, giving rise to the so-called η-invariants. There is also a generalization in [147] which is an improvement of Morita’s work. We define it here only in the classical case, which is all we need in the sequel.

IV.9. The signature of smoothings

129

We recall that if X is a smooth, oriented closed 4-manifold, then the signature theorem of Thom-Hirzebruch says that its signature is given by the Pontryagin number: 1 σ(X) = p1 (X)[X] . 3 Now let X be a compact manifold with non-empty boundary M ; both sides of this equation still make sense, but p1 (X) lives in H 4 (X) ∼ = 0, so one may not expect an equality in general. We notice however that the bundle T X|M is trivial, and a choice of a trivialization τ leads to a representative of p1 (X) that vanishes over M and so defines a relative Pontryagin class p1 (X, τ ) ∈ H 4 (X, M ) ∼ = Z, see §1 above. This class depends on the choice of the trivialization τ of T X|M , so it cannot give the signature of X for arbitrary τ . Notice that the trivialization τ determines an orientation of M , and we assume this is the same as the orientation M gets as being the boundary of X. One has that the difference: dσ (X, τ ) =

1 p1 (X)[X] − σ(X) , 3

is actually independent of X: it depends only on its boundary M and the trivialization τ of the bundle T X|M = T (M ) ⊕ (1) , where (1) denotes the trivial 1-dimensional bundle, isomorphic to the normal bundle of M in X. In fact, suppose we are given another compact, oriented 4-manifold X  with the same boundary M (as oriented manifolds). Then one can form the union Y = X ∪ (−X  ), where (−X  ) is X  with the opposite orientation; Y is a smooth, closed, oriented 4-manifold, and one has: σ(Y ) = σ(X) − σ(X  ) and

 1 1 p1 (Y )[Y ] = p1 (X, τ )[X] − p1 (X  , τ )[X  ] , 3 3

because p1 (Y ) vanishes over M . Hence one has:     1 1 p1 (X)[X] − σ(X) − p1 (X  )[X  ] − σ(X  ) dσ (X, τ ) − dσ (X  , τ ) = 3 3      1 = p1 (X)[X] − p1 (X  )[X  ] − σ(X) − σ(X  ) = 0 . 3 Therefore dσ (X, τ ) depends only on the framed boundary (M, τ ) and not on the choice of X. Since the group of oriented cobordism is trivial in dimension 3 (see, e.g., [244]), it follows that every framed 3-manifold M as above is the boundary of a compact, oriented 4-manifold, with the orientation being compatible with the one defined by τ .

130

Chapter IV. The General Index Theorem

9.1 Definition. The invariant dσ (X, τ ) = 13 p1 (X)[X] − σ(X) is called the signature defect of the framed manifold (M, τ ). We have: 9.2 Theorem. (Durfee) Let (V, 0) be a normal, numerically Gorenstein surface singularity, which is smoothable. Assume further that Vt is a smoothing of V with (topologically) trivial tangent bundle. Then: σ(Vt ) = −

 1 2(χ(Vt ) − 1) + K 2 + s + 2b1 , 3

where b1 is the first Betti number of the resolution and s is the second Betti number. Proof. Observe first that if (V, 0) is numerically Gorenstein, then by Corollary 6.3.iii above we know that the structure group of the bundle T Vt |M can be reduced to SU (2). Since SU (2) ∼ = Sp(1), this means we have multiplication of tangent vectors in this bundle by the quaternions i, j, k. Now let ν denote the unit outwards normal vector field of M in Vt (with respect to some metric). Then multiplying ν by i, j, k at each point of M we obtain three linearly independent vector fields on M , which define a trivialization of T M which is compatible with the complex structure on T Vt |M , in the sense that they determine with ν a trivialization of this bundle as a complex bundle. Now, let M be the link of V and let ρ be a trivialization of T Vt obtained as above, i.e., multiplying by i, j, k a unit normal vector field on M = ∂Vt . Let π : V → V be a resolution of V . By the previous discussion we have: 1 1 p1 (Vt )[Vt ] − σ(Vt ) = p1 (V )[V ] − σ(V ). 3 3 But both T Vt and T V are complex bundles and the trivialization ρ is compatible with their complex structure, so the Pontryagin number can be expressed in terms of the Chern numbers relative to ρ. One has: p1 (Vt )[Vt ] = (c1 (Vt )2 − 2c2 (Vt ))[Vt ] = −2χ(Vt ) because c1 (Vt )2 = 0, since T Vt is trivial by hypothesis, and c2 (Vt )[Vt ] = χ(Vt ) because ρ is given by a parallelism on the boundary M . Similarly, p1 (V )[V ] = K 2 − 2χ(V ) and σ(V ) = −b2 (V ), the 2nd Betti number of V , because the intersection matrix on V is negative definite by Mumford’s theorem (see §4 above). Putting these equations together we arrive at Durfee’s formula.
Notice that Gorenstein singularities are numerically Gorenstein. So Theorem 9.2 applies to these singularities provided the tangent bundle of the smoothing is topologically trivial, a fact conjectured by Durfee in [64, 1.6] and proved in [225]:

IV.10. Rochlin invariant

131

9.3 Proposition. Let Vt be a smoothing of a normal, Gorenstein surface singularity. Then the complex tangent bundle of Vt is topologically trivial,. The proof of 9.3 is immediate from Lemma 6.2 and Proposition 7.4. Thus one has, 9.4 Corollary. Let (V, 0) be a normal, smoothable Gorenstein surface singularity. Then for every smoothing of V and every resolution one has: σ(Vt ) = −

 1 2(χ(Vt ) − 1) + K 2 + s + 2b1 , 3

where these invariants are as in Theorem 9.2.

IV.10

On the Rochlin µ invariant for links of surface singularities

We know (Rochlin’s theorem) from Section 3 that if X is a closed, spin 4-manifold, then its signature is divisible by 16. This allows us to define an invariant of spin 3-manifolds as follows: let (M, S) be a spin 3-manifold. Since the group of spin cobordism is trivial in dimension 3 (see, e.g., [244]), it follows that there exists a compact, spin 4-manifold X whose spin-boundary is (M, S); let µ(M, S) be defined by µ(M, S) = σ(X) mod (16). An argument similar to the one we used in the previous section to define the signature defect shows that this invariant is well defined modulo 16. This is the Rochlin µ invariant of the spin manifold (M, S), which played an important role in 3-manifolds theory several decades ago, and still does, being related to the new 3-manifolds invariants coming from gauge theory (see §11 below). This is of particular interest when M is a homology sphere, since in this case the spin structure on M is unique. Notice that Rochlin’s Theorem 3.7 allows us to compute this invariant by having (M, S) as an oriented boundary, provided we are given a characteristic submanifold (c.f. [224, 231]): 10.1 Definition. Let X be a compact oriented manifold with spin-boundary (M, S). A characteristic submanifold of X relative to the spin structure on the boundary means an oriented 2-submanifold W in the interior of X, representing a homology class in H2 (X; Z) whose reduction modulo 2 is the Poincar´e-Lefschetz dual of the second Stiefel-Whitney class w2 (X, S) ∈ H 2 (X, M ; Z2 ) of X relative to the spin structure on its boundary. Notice that a submanifold W as above actually defines a spinc structure on X which is compatible with the given spin structure on the boundary. Now, given the spin 3-manifold (M, S), consider it as a spinc -boundary of X as above, and let W be a characteristic submanifold of X for this spinc structure.

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Chapter IV. The General Index Theorem

Then one has: 10.2 Theorem. The Rochlin invariant of (M, S) is: µ(M, S) = σ(X) − (W 2 + 8 Arf(W ) )

mod (16) ,

where Arf ∈ {0, 1} is the Arf invariant of a certain quadratic form on H1 (W ; Z2 ). The proof is left as an exercise (using Theorem 3.7). Of course if X is spin, then we can take W = ∅ and we recover the usual definition of the Rochlin invariant. Now consider a normal Gorenstein surface singularity (V, 0) and let Ω be a never-zero holomorphic 2-form on V ∗ = V − 0. Just as before, we see that this 2-form defines a reduction of the structure group of T V ∗ to SU (2) ∼ = Sp(1) ∼ = ∗ Spin(3). Hence it defines a spin structure on V . But it actually gives more: a multiplication by the quaternions i, j, k at each fibre of the tangent bundle T V ∗ , varying smoothly, just as in III.6. If ν denotes a unit outwards, normal vector field of the link M in V ∗ , then multiplication by i, j, k determines a trivialization of the bundle T V ∗ |M compatible with its complex structure.  In factthis is the same 0 1 as multiplying ν at each point by the unitary matrix , which defines −1 0 a section of T V ∗ , everywhere linearly independent of ν over C. We denote this trivialization of T M by ρ. This gives a spin structure on M . Now let π : V → V be a good resolution of V . For convenience we think of V indistinctly as a complex manifold or as a compact almost-complex manifold with boundary the link M and a complex structure in its interior. The trivialization ρ defines Chern classes of V relative to the boundary; they live in H 2i (V , M ; Z). The Poincar´e-Lefschetz dual of c2 (X; ρ) is χ(V ) by construction; the Poincar´e-Lefschetz dual of c1 (X; ρ) is the anti-canonical class −K, and it is independent of the choice of complex trivialization of T V |M , because the intersection matrix on V is negative definite. We call such a parallelism on M (that defines a trivialization of T V |M as a complex bundle) a complex parallelism on M . Thus, if K is non-singular, then K is a characteristic submanifold for X, relative to any spin structure on the boundary which is given by a complex parallelism on M . More generally, choose the canonical divisor K to be vertical, and let W = 2D − K, D vertical, be any characteristic divisor as in §9. Then W represents an integral homology class whose reduction modulo 2 is the dual of w2 (X, ρ). If L is the bundle of W , then we can
defined by a C ∞ section of L which approximate W by a smooth C ∞ manifold W
is transversal to the zero-section. Then W inherits a spin structure from the spinc structure on V : this follows from [192] in the general case, and also from [16] if
is complex analytic, since the line bundle D of D is a theta characteristic for W
. As noted in [231], the Arf invariant in Theorem 10.2 turns out to be a spin W cobordism invariant, so it is 0 iff W (with its induced spin structure) is zero in the group Ω2spin ∼ = Z2 of spin cobordism.

IV.10. Rochlin invariant

133

One has: 10.3 Theorem. Let M be the link of a normal Gorenstein surface singularity (V, 0), let V be a good resolution of V and let β be a parallelism on M compatible with the complex structure on V . Let W = 2D − K be a characteristic divisor of V and
be a C ∞ smoothing of it. Then: let W
) is the Arf invariant in The
) ≡ h0 (W ; D|W ) mod (2), where Arf(W (i) Arf(W orem 10.2, and this is independent of the choice of the complex parallelism
is independent of β. on M . Hence the spin cobordism class of W (ii) In particular Arf(K) is the reduction mod (2) of the geometric genus ρg (V,0). (iii) The Rochlin invariant of M is independent of the choice of the complex parallelism on M and equals: µ(M ) = −s − (W 2 + 8 Arf(W ) )

mod (16) ,

where s is the number of irreducible components in the exceptional divisor of V . (iv) If the singularity (V, 0) is smoothable and Vt is a smoothing of V , then µ(M ) = σ(Vt )

mod (16) .

The proof of statement (i) is an exercise (using (4.9) above); statement (ii) follows from (i) and Theorem 8.1.i. Statement (iii) is obvious from the previous discussion and the last statement follows from Proposition 7.4. It is worth remarking that the Rochlin invariant was studied by several authors for singularities whose link is a homology sphere, specially in [187]. By (ii) above this is essentially an invariant of the singularity (V, 0), independent of the various choices. This invariant was computed in [231] for all quasi-homogeneous singularities and for the cusps of Hirzebruch [104] ( and also for all 3-manifolds of the form Γ\G, where G is a 3-dimensional Lie group and Γ a discrete subgroup). As a corollary one gets an improvement of [225, 4.5]: 10.4 Corollary. Let (V1 , P1 ) and (V2 , P2 ) be normal, Gorenstein surface singularities with orientation preserving links, and let µ(Vi ) denote the corresponding Rochlin invariant. Then:   (i) µ(V1 ) − µ(V2 ) ≡ 8 ρg (V1 ) − ρg (V2 ) mod (16), where ρg is the geometric genus. (ii) If the singularities are smoothable and V1# , V2# are smoothings of them, then:   σ(V1# ) − σ(V2# ) ≡ 8 ρg (V1 ) − ρg (V2 ) mod (16); and   χ(V1# ) − χ(V2# ) ≡ 12 ρg (V1 ) − ρg (V2 ) mod (24) . The first statement above follows from Theorem 10.3 and the fact that, by [184], the topology of the link determines the topology of the minimal resolution, which gives the number of irreducible components in the exceptional divisor

134

Chapter IV. The General Index Theorem

and determines the canonical class K numerically. The second statement follows from the first one, together with Corollary 9.4 above. This motivates the following question, whose answer I do not know. 10.5 Question. Is there an analogous statement to Corollary 10.4 for complex singularities in higher dimensions? In particular, does the fact that two isolated hypersurface singularities have orientation preserving links imply anything about their Milnor numbers? This question was asked of me by V.I. Arnold and I do not know the answer. For instance, the singularities z12 + z27 + z314 = 0 and z13 + z24 + z312 = 0 have orientation preserving links, but their Milnor numbers are 78 and 66 respectively, so the congruences above are best possible. On the other hand one may ask under what circumstances is the geometric genus a topological invariant? (see [181, 182]). 10.6 Remark. There is an invariant of framed cobordism called the real Adams e-invariant eR . For framed 3-manifolds this takes values in Z24 . There is also a complex Adams e-invariant, but this is weaker (though easier to compute). We refer to [224, 231] for details on this and for the relation with complex singularities. From the viewpoint of surface singularities as above, the invariant eR gives essentially the same information as the Rochlin invariant (which takes values in Z16 ); each of these has some advantages, but eR is easier to handle.

IV.11

Comments on new 3-manifolds invariants and surface singularities

As pointed out earlier, since the orientation preserving homeomorphism type of the link M of a normal surface singularity (V, 0) depends only on the analytic type (V, 0), it follows that every 3-manifolds invariant is an invariant of singularities. This has been used by several authors in both ways. On the one hand, whatever invariant we want to understand, the links of surface singularities are a great source of examples which we have more chances of being able to put our hands on. Conversely, the various invariants of 3-manifolds give a lot of information about the singularities themselves. The previous sections are a taste of what is known in that direction concerning more classical invariants. There are however numerous, very important invariants that have been defined in more recent years, and which are just beginning to be studied. Among these are the η-invariants of Atiyah, Patodi and Singer, published in 1975 and 1976 in Math. Proc. Cambridge Philos. Soc. There is one such invariant for each first-order elliptic, self-adjoint operator acting on the sections of a vector bundle over the link of the singularity. Very little is yet known about these (see, for instance, [230, 25, 190, 180]). Related with the eta-invariants, via the spectral flow, is the Floer homology (also called “instanton homology”). This homology HF∗ (Σ) was defined by

IV.11. 3-manifolds invariants and surface singularities

135

A. Floer [79] for every integral homology sphere, as the homology of a Z8 -graded complex generated by the critical points of a perturbed Chern-Simons functional (on a certain space of connections). The Casson invariant λ(Σ) is essentially the alternating sum of the Betti numbers of the Floer homology, and it is an integral lifting of the Rochlin invariant, i.e., λ(Σ) is an integer whose reduction modulo 16 gives the Rochlin invariant. For homology 3-spheres which are links of surface singularities, the signature of the Milnor fibre provides another integral lifting of the Rochlin invariant, and the so-called Casson invariant conjecture [188] states that these two liftings of the Rochlin invariant should coincide, i.e., that for homology 3-spheres which are links of surface singularities, the Casson invariant λ(Σ) should equal 18 σ(F ), an eighth of the signature of the corresponding Milnor fibre. I am grateful to W.D. Neumann for explaining to me that this conjecture was first suggested by J. Wahl to M.F. Atiyah at the 1987 Weyl conference at Duke in response to Atiyah asking if there were connections between the analytic structure of singularities and gauge-theoretic invariants; that led Atiyah to subsequently make a conjecture about Floer homology. Fintushel and Stern studied in [78] the Floer homology of the Brieskorn homology spheres Σ(p, q, r) and proved the Casson invariant conjecture for these; some of the calculations in [78] were extended in [84] to Seifert fibred homology 3-spheres in general. In [188] and [83], independently, the authors proved the Casson invariant conjecture for all homology spheres given by weighted homogeneous surface singularities (in [188] they proved it also for other families of singularities). There is also a very interesting article by N. Saveliev [219], where the author relates in a precise way the Floer homology of Brieskorn homology spheres with the topology of the Milnor fibre and with classical invariants of 4-manifolds and knots. But the Casson invariant conjecture remains open. Recently, in a beautiful set of lectures delivered by W.D. Neumann in a workshop at Luminy, France, he spoke about his work in process with J. Wahl in this direction, and about their new “Milnor fibre conjecture”, which implies the Casson invariant conjecture. In fact, since 1982 much of the progress in low-dimensional topology has arisen from applications of gauge theory, pioneered by S.K. Donaldson. In particular, Donaldson’s polynomial invariants have been used to prove a variety of results about the topology and geometry of 4-manifolds. Kronheimer and Mrowka showed in [115] that there is a deep structure encoded in the Donaldson invariants which is related to embedded surfaces in 4-manifolds; this was a significant step for understanding these invariants, and it was a motivation for important further developments, most spectacularly for Witten’s introduction of the now-called Seiberg-Witten (SW) monopole equations for the study of four-manifolds. These 4-dimensional SW equations yield differentiable invariants that have all the power of the polynomial invariants of Donaldson, but they are much easier to handle; these are known as the Seiberg-Witten invariants of smooth 4-manifolds. There are also the 3-dimensional Seiberg-Witten equations, giving rise to the Seiberg-Witten invariants of 3-manifolds, whose understanding is important also for 4-manifolds 0 (σ) to each spinc structheory. These invariants associate a rational number swM

136

Chapter IV. The General Index Theorem

ture σ on a 3-manifold M . For homology spheres there is only one such structure and the Seiberg-Witten invariant coincides with the Casson invariant (see [142]). When M is the link of a surface singularity (V, 0), the complex structure on V − 0 determines a canonical spinc structure on M and the remarkable articles of A. N´emethi and L. Nicolaescu [181, 182, 190, 180] are throwing light into the relation of the corresponding SW invariants with other classical invariants of surface singularities, particularly with the geometric genus. The hope is to find topological invariants of singularities, which determine the analytic ones. Their results led them to make a conjecture, the Seiberg-Witten invariant conjecture, which implies the Casson invariant conjecture, and they managed to prove it in some cases. However the recent article [77] provides counterexamples to this more general conjecture (but the original Casson invariant conjecture of [188] remains alive!).

Chapter V

On the Geometry and Topology of Quadrics in CPn In Chapter II we saw how the action of the group SU (2) on C2 gives very precise information about certain surface singularities; in particular we described the close relation between SO(3, R) = SU (2)/ ± Id and the quadric z12 + z22 + z32 = 0. In this chapter, which is based on [135], we look at the canonical action of SO(n + 1, R) on Cn+1 and on CPn , the complex projective space, in order to get a better understanding of the geometry and topology of the pair (Cn+1 , V ), where V is the Fermat quadric z02 + z12 + · · · + zn2 = 0. This is of course related to the classical problem studied by Zariski [267] and others, of studying the topology of the complement of an affine algebraic hypersurface V ⊂ Cn+1 . We actually look with more care at the projectivized situation. We notice that the complement of a non-singular hyperquadric Q in CPn is diffeomorphic to the total space of the tangent bundle of the real projective n-space RP n , CPn − Q ∼ = T (RP n ) . Then we use this observation to describe CPn as the double mapping cylinder of the double fibration:

Q where

F+n+1 (2, 1) II v IIπ2 π1 vvv II v v II vv I$ v { v

PRn

F+n+1 (2, 1) ∼ = SO(n + 1, R)/(SO(n − 1, R) × (Z/2Z))

is the partial flag manifold of oriented 2-planes in Rn+1 and non-oriented lines in these planes. The manifold F+n+1 (2, 1) is diffeomorphic to the unit sphere normal

138

Chapter V. Geometry and Topology of Quadrics

bundle of Q in CPn , and it is also diffeomorphic to the unit sphere tangent bundle of PRn . This decomposition is related to previous work by V. Vassiliev, J. Tits, C.T.C. Wall and others, and we refer to [135] for details and for more on the subject. In Section 3 we look more carefully at the decomposition of CPn arising from the above double fibration. This describes CPn as a 1-parameter family of codimension 1 submanifolds F+n+1 (2, 1) × {t}, for t ∈ (0, 1), together with two “special” fibres: Q and a copy of the real projective space. We prove that these are the orbits of the natural action of SO(n + 1, R) on CPn , regarded as a subgroup of the complex orthogonal group SO(n + 1, C). This is a cohomogeneity 1 isometric action with respect to the Fubini-Study metric. In Section 4 we look at the (now classical) theorem saying that CP2 modulo conjugation is the sphere S4 . This theorem has a long and remarkable history. The first time this appeared in print (without proof) was in [8, p. 175], where Arnold used it to study real algebraic curves in RP 2 . In 1973–74 appeared two independent proofs of this theorem, given by W. Massey and N. Kuiper [116, 159]. Several other proofs of this result have been given by various authors afterward, including important improvements and generalizations (see for instance [10, 11, 12, 22, 153, 175]). We refer particularly to the recent article of M. Atiyah and J. Berndt [17], whose proof is along the same lines of the one we describe here (which is taken from [135]), but they do it in a more general setting. Here, as in [17], we prove an equivariant version of the Arnold-Kuiper-Massey theorem, showing that the equivalence CP2 /j ∼ = S4 can be realized by a real algebraic map Φ which conjugates the natural cohomogeneity 1-actions of SO(3, R) on CP2 and S4 . Our proof uses only linear algebra.

V.1

The topology of a quadric in CPn

Let h be a homogeneous polynomial of degree 2 in n + 1 complex variables with  = h−1 (0), K = Q  ∩ S2n+1 be its link, an isolated critical point at 0 ∈ Cn+1 ; let Q n ∗  − 0)/C , which is non-singular and let Q ⊂ CP be its projectivization, Q = (Q by hypothesis. 1.1 Theorem.  is isotopic to the hypersurface given by the Fermat poly(i) The hypersurface Q nomial equation z02 + · · · + zn2 = 0. The analogous statement holds for the projectivized quadrics. (ii) The Milnor fibre F of h is diffeomorphic to the total space of the tangent bundle of the n-sphere Sn . Hence the link K is diffeomorphic to the total space of the unit sphere tangent bundle of Sn . Therefore K is diffeomorphic to the Stiefel manifold Vn+1,2 of orthonormal 2-frames in Rn+1 . (iii) The quadric Q ⊂ CPn is diffeomorphic to the Grassmannian Gn+1,2 of oriented 2-planes in Rn+1 .

V.1. The topology of a quadric in CPn

139

iv) The complement CPn − Q is diffeomorphic to the quotient space F/Z2 of the Milnor fibre F by the monodromy of the Milnor fibration, which is cyclic of order 2. Hence CPn − Q is diffeomorphic to the total space of the real projective space RP n . Proof. The first statement in Theorem 1.1 actually holds for homogeneous polynomials of any degree d > 1. To prove this, let P be the projective space of coefficients of homogeneous polynomials of degree d in n + 1 complex variables. The general homogeneous equation of degree d in n + 1 variables is:  aαo ,...,αn z0αo · · · znαn = 0 . αo +···+αn =d

This defines a polynomial, and hence a hypersurface X , in P × CPn . The family of projective hypersurfaces of degree d in CPn is given by the map: E :X →P, induced by the projection of P × CPn onto P. In P, the polynomials defining singular hypersurfaces in CPn form a closed subvariety of complex codimension 1. Hence its complement Ω is connected. Since the map E is a locally trivial fibration over Ω, by Ehresmann’s Lemma (Chapter I), one knows that any nonsingular hypersurface X ⊂ CPn of degree d is ambient isotopic to the hypersurface defined by the Fermat polynomial Fdn := z0d + · · · + znd . That is, up to isotopy we can assume that X is the projectivization of the affine variety V := {z0d + · · · + znd = 0} after removing the singular point 0 ∈ V (c.f. [133, Lemme 2.2]). The corresponding statement for the affine hypersurfaces follows easily from this, proving the generalization of statement (i) for polynomials of degree d. Also notice that CPn is the orbit space of Cn+1 − {0} by the C∗ -action: gt (zo , . . . , zn ) = (tzo , . . . , tzn ) , t ∈ C∗ = C − {0} . If V is as above, then V is an invariant set for this C∗ -action. It follows that CPn − X is the image of Cn+1 − V . Moreover, C∗ is S1 × R+ and if we divide Cn+1 − {0} by the R+ -action we get the sphere S2n−1 . Thus CPn − X is the quotient of S2n−1 − (V ∩S2n−1 ) by the corresponding S1 -action. By [168], these S1 orbits are transversal to the Milnor fibres of the polynomial Fdn (z) = z0d + · · · + znd , and their action on the fibres is given by the monodromy of the Milnor fibration, which is cyclic of period d. Therefore the Milnor fibre F is a d-fold cyclic cover of CPn − X, showing that the first statement in (iv) also holds for hypersurfaces of degree d > 1. In the quadratic case d = 2 the Milnor fibre is diffeomorphic to the affine variety z02 + · · · + zn2 = 1. Let us decompose each vector Z := (z0 , . . . , zn ) in its real and imaginary parts, Z = U + iV ; then the Milnor fibre is given as the set (U, V ) ∈ Rn+1 × Rn+1 such that |U |2 − |V |2 = 1 and U ⊥ V . We notice that the map (U, V ) → (U/U , V ) induces an isomorphism of this Milnor fibre F with

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Chapter V. Geometry and Topology of Quadrics

the tangent bundle of Sn , proving the first statement in (ii). Of course we may think of F as being the unit disc tangent bundle of the sphere. Hence the link K is the unit sphere tangent bundle of Sn , since K can be regarded as the boundary of F . Therefore each point in K corresponds to a point in Sn , i.e., a unit vector in Rn+1 together with a unit vector orthogonal to the first one. Hence K is the Stiefel manifold Vn+1,2 , as claimed in (ii). Statement (iii) follows easily from statement (ii). In fact, it is clear that (V − 0)/C∗ is diffeomorphic to K/S1 . The link K is Vn+1,2 and the corresponding S1 ∼ = SO(2)-action identifies all 2-frames which are in the same 2-plane in Rn+1 . Hence K/S1 is the Grassmannian Gn+1,2 . To complete the proof of Theorem 1.1 it only remains to prove the last statement in (iv). From the previous discussion we know already that CPn − Q is diffeomorphic to the quotient of F , the Milnor fibre, by the monodromy, which is cyclic of order 2. We also know that F is the total space of the tangent bundle T (Sn ). We observe that, using the previous notation, the monodromy is given by multiplication by −1, (U, V ) → (−U, −V ). Hence the quotient of F by this involution is the tangent bundle of the real projective n-space.
We notice that part of the argument above is similar to that of Lemmas 2.2 and 2.3 in [133] (see also Libgober in [139, Lemma 1.1]), implying Corollary 1.2 below. We denote by X0 the projectivization of the affine hypersurface defined by the Fermat polynomial Fdn := z0d + · · · + znd , and we denote by Cdn := CPn − X0 the complement of X0 . 1.2 Corollary. Let X be a non-singular hypersurface of CPn of degree d. Then: (i) the pair (CPn , X) is isotopic to the pair (CPn , X0 ); and (ii) the Milnor fibre F of Fdn is a d-fold cyclic cover of Cdn , the projection map F → Cdn being given by the monodromy of the Milnor fibration of Fdn (which is cyclic of period d). Since by [198, 168] the Milnor fibre has the homotopy type of a bouquet S n of µ spheres Sn , where µ = (d − 1)n+1 is the Milnor number, one has (as in [139]) that for n > 1 the fundamental group π1 (Cdn ) is isomorphic to Z/dZ and n ∼ πj (Cd ) = πj ( µ S n ) for j > 1. In particular:

µ

πj (Cdn ) ∼ = 0 if 1 < j < n

and

πj (Cdn ) ∼ = Zµ if j = n .

(1.3)

1.4 Remark. It should be noted that (1.3) is a special case of the results proved by A. Libgober in [139] about the topology of the complement of projective hypersurfaces. There are also interesting results in the recent article [15] about the topology of non-singular complete intersections in complex projective spaces.

V.2. The space CPn as a double mapping cylinder

141

V.2 The space CPn as a double mapping cylinder From now on we let Q ⊂ CPn be the non-singular hyperquadric in CPn with equation z02 + · · · + zn2 = 0, in homogeneous projective coordinates. Let j : CPn → CPn be the involution on CPn given by complex conjugation: j([z0 , . . . , zn ]) = [¯ z0 , . . . , z¯n ], and let Π be the fixed point set of j, so that Π ∼ = PRn . Theorem 1.1 says that CPn − Q is diffeomorphic to the tangent bundle T (Π), and Π is the zero section of this bundle. Hence CPn − (Q ∪ Π) can be regarded as the set of non-zero tangent vectors of Π, so it is diffeomorphic to the cylinder T1 (Π) × (0, 1), where T1 (Π) is the unit sphere tangent bundle. In other words, we have that CPn is obtained by taking the product T1 (Π) × (0, 1) and attaching to it in some way the quadric Q on one end and the real projective space Π = RP n on the other end. We now explain these “attaching” functions. For this we have: 2.1 Proposition. The unit sphere tangent bundle T1 (Π) is diffeomorphic to the homogeneous space SO(n + 1, R)/(SO(n − 1, R) × Z2 ), and it is therefore diffeomorphic to F+n+1 (2, 1), the (partial) flag manifold of oriented 2-planes in Rn+1 and unoriented lines in these planes. Proof. Notice that the group SO(n + 1, R) acts linearly on Cn+1 and this action descends to an action on CPn that preserves Q. This action also leaves invariant the real projective space Π, where it acts in the usual way (i.e., via the action induced from the linear SO(n + 1, R)-action on Rn+1 ). This extends, via the differential, to a transitive action of SO(n + 1, R) on T1 (Π), with isotropy subgroup SO(n − 1, R) × Z2 . Hence T1 (Π) is diffeomorphic to SO(n + 1, R)/(SO(n − 1, R) × Z2 ), as claimed in Proposition 2.1. But SO(n + 1, R) also acts transitively on F+n+1 (2, 1) with isotropy SO(n − 1, R) × Z2 . Hence: T1 (Π) ∼ =

SO(n + 1, R) ∼ = F+n+1 (2, 1) , SO(n − 1, R) × Z2


as stated. F+n+1 (2, 1)

consists of an Now observe that each point in the flag manifold oriented plane in Rn+1 and a line in this plane. If we forget the 2-plane and keep the line, we get an obvious map from F+n+1 (2, 1) into RP n , which is actually a fibration. Similarly, we may forget the line and keep the 2-plane, getting a projection into the Grassmannian Gn+1,2 , which is diffeomorphic to the quadric Q, by Theorem 1.1. Thus one has a double fibration: F+n+1 (2, 1) JJJ v π1 vvv JJπJ2 v JJJ vv v J% zvv Q RP n

(2.2)

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Chapter V. Geometry and Topology of Quadrics

where π1 and π2 are the maps that assign to each flag (P, l) either the 2-plane P ∈ Gn+1,2 or the line l ∈ RP n .   We form the corresponding double mapping cylinder F+n+1 (2, 1) × [0, 1] / ∼ where ∼ identifies a point ((P0 , l0 ), 0) ∈ F+n+1 (2, 1) × {0} with the point π1 (P0 , l0 ) = P0 in Gn+1,2 ∼ = Q, and a point ((P1 , l1 ), 1) ∈ F+n+1 (2, 1) × {1} with the point π2 (P1 , l1 ) = l1 ∈ RP n . The space we obtain is homeomorphic to CPn . Furthermore, the double fibration (2.2) splits into two fibrations, corresponding to the maps π1 and π2 . The open mapping cylinder of π1 is CPn − Π, while the open mapping cylinder of π2 is CPn − Q. One has: 2.3 Theorem. The projective space CPn is the double mapping cylinder of the double fibration (2.2). If we remove Q from CPn we obtain a manifold diffeomorphic to the total space of the normal bundle of Π ∼ = PRn in CPn . Reciprocally, if we remove n Π from CP , what we get is diffeomorphic to the total space of the normal bundle of Q in CPn . If we remove Q ∪ Π from CPn , what we get is diffeomorphic to F+n+1 (2, 1) × (0, 1). Proof. We notice that if we replace in Theorem 2.3 the word diffeomorphic by homeomorphic, then this theorem follows immediately from the previous discussion. Let us prove that we actually have diffeomorphisms. By Theorem 1.1, this is clear for CPn − Q. In fact, the fibration of CPn − (Q ∪ Π) given by the manifolds F+n+1 (2, 1) corresponds to the fibration on T (Π) − Π given by sphere bundles of radius r > 0, for some metric on T (Π). These correspond to boundaries of tubular neighborhoods ν˜r (Π) of Π ⊂ CPn . In particular CPn −Q is a tubular neighborhood of Π, hence CPn − Q is diffeomorphic to the total space of the normal bundle of Π∼ = PRn in CPn . This bundle is isomorphic to T (Π). Let us prove that CPn − Π is diffeomorphic to the total space of the normal bundle of Q in CPn . We observe that for all r > 0, the interior of CPn − ν˜r (Π) is diffeomorphic to CPn − Π. Now we prove that CPn − Π is actually a tubular neighborhood of Q. For this we recall that one has the Fubini-Study metric on CPn , which can be thought of as being the descent to CPn ∼ = S2n+1 /S1 of the usual 1 metric on the sphere, which is S -invariant. If N is a Riemannian submanifold of CPn , its normal map NN is the function that associates to each normal vector v of N the projection to CPn (via the exponential map) of the end-point of v ∈ T (CPn ) (see, for instance [169], p. 32). Let us denote by ν(Q) the normal bundle of Q in CPn and consider the normal map: NQ : ν(Q) → CPn . We notice that every complex projective line L in CPn orthogonal to Q, for the Fubini-Study metric, is invariant under conjugation, which is an isometry. So L is

V.3. The orthogonal group SO(n + 1, R) and CPn

143

defined by equations with real coefficients (c.f. Section 3 below), and it is totally geodesic in CPn since it is a complex projective line. Therefore L intersects Π transversally in a real projective line. This implies that the normal map NQ is a diffeomorphism from the open disk bundle in ν(Q) of radius π2 into CPn − Π. The union of all closed geodesic segments normal to Q of length π2 fill up all of CPn . Thus the distance from a point p ∈ CPn − (Π ∪ Q) to Q is exactly the length of the unique geodesic segment joining p and the unique point q ∈ Q such that this segment is orthogonal to Q. Hence every tubular neighborhood of Q in CPn , of diameter less than π2 , is diffeomorphic to CPn − Π.
We remark that one has a construction for the Milnor fibre F of the Fermat polynomial F2n in the spirit of Theorem 2.3, since F can be regarded as the open mapping cylinder of the fibration Vn+1,2 ∼ = SO(n + 1, R)/SO(n − 1, R) −→ SO(n + 1, R)/SO(n, R) ∼ = Sn , where Vn+1,2 is the aforementioned Stiefel manifold.

V.3 The orthogonal group SO(n + 1, R) and the geometry of CPn We now look more carefully at the decomposition of CPn arising from the double fibration (2.2). It is convenient to look at two other interesting foliations that arise naturally from the double fibration (2.2), and from other considerations too. 3.1 Proposition. The double fibration (2.2) induces two foliations F1 and F2 such that: (i) The first foliation F1 is defined on CPn − Π; its leaves are embedded copies of R2 orthogonal to Q, which are the image under the normal map of Q of the fibres of the normal disc bundle of Q of radius less than π2 . The closure of each such leaf is a closed 2-disc that meets Π orthogonally in a projective line which is a closed geodesic in CPn . For each pair of conjugate points in Q, the corresponding leaves are naturally glued together along their common limit set in Π, forming a complex projective line defined by real coefficients. (ii) The second foliation F2 is defined on CPn − Q; its leaves are embedded ndiscs, orthogonal to Π, which are the image under the normal map of Π of the fibres of the normal disc bundle of Π of radius less than π2 . The closure of each such leaf is a closed n-disc that meets Q orthogonally in an (n − 1)-sphere, invariant under complex conjugation. Here, by the limit set of a leaf L we mean the difference L − L, where L is its topological closure. Proof. The leaves of the first foliation F1 on CPn − Π are the fibres of π1 , which are 2-discs transversal to Q, by Theorem 2.3. By construction, each leaf of F1 is

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transversal to all the manifolds F+n+1 (2, 1) × t ⊂ CPn for t ∈ (0, 1), intersecting each in a copy of RP 1 and approaching Π as t → 1. Let us construct this foliation in a different way, which actually gives even more information than what is stated in Proposition 3.1. Recall one has on CPn the Fubini-Study metric. From the proof of Theorem 2.3 we know that the normal map NQ of Q induces a diffeomorphism between the open disc bundle of radius π/2 and CPn − Π. The leaves of F1 are the images of the normal discs. Since the conjugation j : CPn → CPn is an isometry, we have that a projective line L in CPn intersects Q at two conjugate points iff it is orthogonal to Q, and this happens iff L can be defined by equations with real coefficients. So we call these CR-lines. If two distinct CR-lines intersect, they do so in a point in Π ∼ = PRn . Also, each CR-line L meets Π in a real projective line, which is an equator of L. Since all complex lines in CPn are totally geodesic, the real projective line L ∩ Π is a geodesic in CPn , at equal distance π/2 from both intersection points in L ∩ Q. This divides L into two round discs of maximal diameter, orthogonal to Q. One can prove that through each point in CPn − Π passes a unique CR-line, hence these lines foliate this space. Therefore the open discs into which the CR-lines split fill out the whole of CPn − Π, they are totally geodesic in CPn and orthogonal to Q, thus providing a fibre bundle decomposition of CPn − Π, equivalent to the open disc bundle of the normal bundle ν(Q) of Q in CPn . By construction, the closure of each leaf in CPn is obtained by attaching to the leaf a real projective line RP 1 ⊂ Π, which is its boundary (or limit set). This circle (a real projective line in Π) is invariant by conjugation and it is an equator of the unique CR-line, therefore it is also a closed geodesic for the Fubini-Study metric of CPn . In the case of the foliation F2 , the leaves are the fibres of π2 , up to isotopy. They are transverse to F+n+1 (2, 1) × t, for every t ∈ (0, 1), and these leaves are also transverse to Π. We can describe this foliation, more precisely, as follows. Given z ∈ Π, we let Pz be the pencil of real projective lines in Π passing through z. Note that the tangent vectors at z to the lines of this pencil give the tangent space of Π at z. Let lz be one of the lines of the pencil Pz . Its complexification is a projective line Lz in CPn defined by an equation with real coefficients, invariant under conjugation. This implies that Lz intersects Q at two points w1 and w2 , which are conjugate; the intersection Lz ∩ Q is necessarily orthogonal and lz is an equator in Lz . Thus, there is a segment ˆlz , half of a real projective line (a circle) in Lz , joining the points w1 , z and w2 . This line is orthogonal to Π and to Q, it is geodesic in CPn and has length π, by the minimality of Lz . Doing this for all lines in the pencil Pz , we get an open n-disc of radius π/2 in CPn , orthogonal to Π at z, filled by geodesics in CPn of length π/2 and intersecting Q orthogonally. Thus the normal map NΠ is regular for vectors of norm < π/2. The leaves of F2 are the image under NΠ of the fibres of the open disc normal bundle of Π ⊂ CPn of radius π2 . There is another interesting way of thinking about this foliation, up to isotopy, which is helpful to understand the way how its leaves approach Q. By Corol-

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lary 1.2 we have that CPn − Q is the Milnor fibre F := {z02 + · · · + zn2 = 1} divided by the monodromy (z1 , . . . , zn ) → (−z1 , . . . , −zn ). The fibre F is the tangent bundle of the n-sphere, so it has a natural foliation by leaves diffeomorphic to n-planes. These planes can be described as follows. Let us decompose each Z := (z1 , . . . , zn ) in its real and imaginary parts, Z = U + iV . The fibre F is the set (U, V ) ∈ Rn+1 × Rn+1 such that U  ≥ 1, U 2 − V 2 = 1 and U ⊥ V . If U  = 1, then we are on the n-sphere and V  = 0. Given a fixed Uo ∈ Sn ⊂ Rn+1 , its “tangent space” is the plane defined as follows: for each λ ∈ R with λ > 1, let Sλ (U0 ) be the (n − 1)-sphere in the affine n-plane perpendicular to λUo , consisting of all vectors V such that the vector Z = λUo + iV is in F ; these must satisfy V 2 = λ2 − 1. The radius of the sphere Sλ (U0 ) grows with λ, while for λ = 1 the corresponding “sphere” is just one point. For a given U0 ∈ Sn , let us denote by L(U0 ) the union of all these (n − 1)-spheres Sλ (U0 ), for all λ ≥ 1. Then L(U0 ) is a copy of Rn embedded in F as a component of the 2-sheeted hyperboloid consisting of L(U0 ) ∪ L(−U0 ). The monodromy map interchanges these two sheets of the hyperboloid, so their image in CPn is a manifold diffeomorphic to a plane, that we denote by F (U0 ). By the uniqueness of the tubular neighbourhood, these are the leaves of F2 up to isotopy. From this description of F2 one can see the way the leaves approach Q. In fact, let us denote by Sλ (U0 ) the image of the sphere Sλ (U0 ) in CPn . It lies in F (U0 ). Let γλ (U0 ) be the intersection of the unit sphere S2n+1 ⊂ Cn+1 with the real half cone over Sλ (U0 ) with vertex at 0. The image of γλ (U0 ) in CPn is also Sλ (U0 ). The sphere γλ (U0 ) is the set of vectors ( √2λλ2 −1 U0 , √2λ12 −1 V ) with (λU0 , V ) in Sλ (U0 ). Therefore the limit of γλ (U0 ) is the set of vectors ( √12 U0 , √12 v) where v is V /V , with V as above. Since the vectors √12 U0 and √12 v have equal length, the image Λ(U0 ) in CPn of this limit set is in Q, and it is an (n − 1)-sphere. By continuity, the limit set of Sλ (U0 ) in CPn is also Λ(U0 ). Since the conjugate of the vector (U, V ) is (U, −V ), the sets γλ (U0 ) and their limit, are invariant under conjugation. Hence Λ(U0 ) is also invariant by conjugation. We notice that the previous discussion also proves the following fact, that we state as a proposition. We recall that given a Riemannian submanifold N of CPn , its focal points are the critical values of the normal map of N , see [169]. ∼ P n , consisting of the points in 3.2 Proposition. The real projective space Π = R n CP with homogeneous real coordinates, is the set of focal points of the quadric Q defined by the Fermat polynomial z02 + · · · + zn2 = 0. Conversely, the quadric Q is the set of focal points of Π. Thus, both manifolds Q and Π can be regarded as caustics in CPn , i.e., they are the critical values of the corresponding co-normal maps of Π and Q, respectively (see [13]). Let us consider now the action of SO(n+1, R) on CPn , regarded as a subgroup of the complex orthogonal group O(n + 1, C). This action leaves Q invariant and

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it is by isometries with respect to the Fubini-Study metric. An isometry of CPn that leaves Q invariant necessarily carries the set of focal points of Q into itself. Hence Π is also an invariant set for the action of SO(n + 1, R). We know already that Q is the Grassmannian Gn+1,2 ∼ = SO(n + 1, R)/(SO(n − 1, R) × SO(2, R)), so the action of SO(n + 1, R) is transitive on Q. Thus Q is one single orbit, and so is Π. Let us look at the orbit of a point w ∈ CPn − (Q ∪ Π). We claim that its orbit is the manifold (F+n+1 (2, 1) × t) passing through w. For this we use again the normal map: NQ : ν(Q) → CPn . By the previous discussion, this map is a diffeomorphism from the open disc bundle in ν(Q) of radius π2 into CPn − Π and the image of the fibres are the leaves of F1 . Hence each point w ∈ CPn − (Q ∪ Π) is in the image of the normal map NQ , i.e., there is a (unique) vector vw ∈ ν(Q) normal to Q, such that w = NQ (vw ); the norm of vw equals the distance dw = d(w, Q) from w to Q, which is > 0 and < π/2. That is, w corresponds, via NQ , to a point in the sphere bundle Sdw (ν(Q)) of radius dw in ν(Q). We claim that the SO(n+ 1, R)-orbit Ow of w is the image of this sphere bundle, i.e., Ow = NQ (Sdw (ν(Q))). For this we notice that the group SO(n + 1, R) also acts on the tangent bundle T (CPn ) via the differential, and this action preserves the (C ∞ ) splitting T (CPn )|Q ∼ = T Q ⊕ ν(Q). This induces an action of SO(n + 1, R) on the normal bundle ν(Q) of Q, and this action is isometric and commutes with NQ , proving the claim. Hence the SO(n + 1, R)orbits are all manifolds (F+n+1 (2, 1) × t), for some t ∈ (0, 1), with two exceptional orbits which are Q and Π, corresponding to t = 0 and t = 1. By [106, 1.1], this implies that Q and Π are minimal submanifolds of CPn , which is obvious for Q, being a complex submanifold. The orbits of maximal dimension, which in this case are diffeomorphic to F+n+1 (2, 1), are called principal orbits. The previous arguments also show that each SO(n + 1, R)-orbit in CPn is at constant distance from Q, and also from Π, and these distances go from 0 to π2 . This proves that the space of SO(n + 1, R)-orbits in CPn is the interval [0, π2 ], with the two special orbits corresponding to the end-points of the interval. But one can actually be more precise about this statement. Let us consider again the geodesic ˆlz described above, in the construction of the foliation F2 . In fact we are interested in half of this geodesic segment. To construct this “half geodesic segment”, that we shall denote by ˇl, we can start with any complex projective CR-line L. This line intersects Π in a real projective line, and it meets Q orthogonally at two conjugate points, say w and w. ¯ Now we choose a point z0 ∈ Π ∩ L. Then ˇl is the π geodesic (of length 2 ) in L joining the points z0 and w, and it is a geodesic in CPn because L is totally geodesic. This geodesic ˇl starts at z0 ∈ Π and finishes at w ∈ Q. Hence it meets each SO(n + 1, R)-orbit orthogonally in exactly one point, since the orbits are the level sets of the function distance to Π. Hence ˇl parametrizes the orbits of SO(n + 1, R). This shows that the SO(n + 1, R)-action on CPn is a hyperpolar isometric action of cohomogeneity 1, which is already well known (see for instance [98]). In fact, cohomogeneity 1 means that the principal

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orbits have codimension 1, and we know that this happens in our case (see the following section). An isometric action is said to be polar if there exists a closed, connected submanifold Σ that meets all orbits orthogonally. In our case this can be, for instance, the complete geodesic in L determined by ˇl. Such a manifold is called a section. If one can chose such a section to be also flat, one says that the action is hyperpolar. This is obviously satisfied in our case since the section is a geodesic. We have thus proved the following: 3.3 Theorem. (i) The natural SO(n + 1, R)-action on CPn is isometric, hyperpolar of cohomogeneity 1, with space of orbits the interval [0, π/2]. A section for this action (i.e., a submanifold that intersects transversally each orbit at exactly one point) can be constructed by considering some (any) CR-line L, choosing a point z ∈ L ∩ Π and taking the geodesic (a circle) in L that passes through z and the two points where L meets Q. (ii) There are three orbit types: two special orbits, Q and Π, which correspond to the endpoints {0, π/2}, and the principal orbits, which are copies of the partial flag manifold, F+n+1 (2, 1) ∼ = SO(n + 1, R)/(SO(n − 1, R) × Z2 ) , of oriented 2-planes in Rn+1 and lines in these planes. The manifold F+n+1 (2, 1) is diffeomorphic to the unit sphere normal bundle of Q in CPn , and also diffeomorphic to the unit sphere tangent bundle of PRn . Each of the two special orbits is the set of focal points of the other, and they are minimal submanifolds of CPn . (iii) The complex projective lines in CPn whose homogeneous coordinates are real, i.e., the CR-lines, foliate CPn − Π and they are everywhere transversal to the orbits of SO(n+1, R) (away from Π). In particular, they are orthogonal to Q. (iv) The real projective space Π ∼ = PRn is embedded in CPn so that its normal bundle is isomorphic to its tangent bundle. Its “tangent spaces” naturally define a foliation of CPn − Q by embedded copies of Rn , which are everywhere transversal to the orbits of SO(n + 1, R) (away from Q). In particular, they are orthogonal to Π. 3.4 Remark. Let q : CPn → [0, π/2] ⊂ R be the function q(Z) = [d(Z, Q)]2 , i.e., q is the square of the distance to Q. It is clear that q is constant along the SO(n + 1, R)-orbits, which are its level sets. Hence q has the two special orbits Q and Π as critical set. It is clear that if Σ is a small disc in CPn orthogonal to Q (or to Π), then the restriction of q to Σ is the ordinary quadratic map, so it is a Morse function on Σ. This means, by definition, that q is a Morse-Bott function (c.f. [33, 61]).

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Cohomogeneity 1-actions of SO(3) on CP2 and S4

We recall that a smooth manifold is said to be a homogeneous space if there is a Lie group that acts transitively on it. More generally, given a connected Lie group G, a smooth, connected manifold M and an action Φ : G×M → M , one has that the Gorbits of highest dimension form a dense open set in M ; these are called principal orbits while the others are the exceptional orbits (see for instance [106]). If the principal orbits have codimension k we say that the action has cohomogeneity k. If G and M are compact, cohomogeneity 1-actions have a simple global structure: either there are no exceptional orbits and the space of orbits is a circle, or else there are just two exceptional orbits and the quotient is the closed unit interval, with the exceptional orbits corresponding to the end-points of the interval. See for instance [175, 98]. In the previous sections we studied an example of such a cohomogeneity 1-action of SO(n + 1, R) on CPn , whose principal orbits were the boundaries of tubular neighborhoods of the quadric Q, or equivalently of the real projective space RP n
→ CPn . This was a case where we had two exceptional orbits: the quadric and RP n . When n = 2 the principal orbits are copies of the flag manifold F+3 (2, 1) and the quadric Q is diffeomorphic to the 2-sphere, since it is the Grassmannian of oriented 2-planes in R3 (by Theorem 1.1 above), and every such plane corresponds uniquely to a point in S2 . We notice that this action is very similar to a classical cohomogeneity 1action of SO(3) on the sphere S4 , studied by Hsiang and Lawson in [106, Example 1.4], and by several other authors. To recall this action it is convenient to think of the sphere in a non-canonical way that we describe now. Let S be the vector space of real 3 × 3, traceless and symmetric matrices. As a real vector space S is R5 , and it can be equipped with a metric given by the inner product (A, B) → trace(AB). Let S(4) be the space of matrices in S with norm 1. One has an obvious diffeomorphism S4 ∼ = S(4) , which becomes isometric 4 if we endow S with its usual round metric and S(4) with the metric given by the inner product in S. We shall identify these two spaces in the sequel, denoting both of them by S4 indistinctly. The group SO(3, R) acts on S by A → Ot AO, where Ot is the transposed matrix (which is equal, in our case, to O−1 ). This induces an isometric action Γ of SO(3, R) on S4 . This action on S4 has two disjoint copies of RP 2 as special fibres. The space of orbits is the interval [0, 1], with the endpoints giving the special orbits. Each principal orbit (i.e., the orbits of highest dimension) is a flag manifold: F 3 (2, 1) ∼ = SO(3, R) / (Z2 × Z2 ) ∼ = L(4, 1) / Z2 , of pairs (P, l) with P a plane in R3 and l line in P , where L(4, 1) is the lens space S3 / (Z/4Z) ∼ = SO(3, R) / Z2 . We notice that this decomposition of the sphere S4 into the orbits of the SO(3)-action is very similar to that of CP2 described previously. In fact this

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similarity is even closer than we have seen so far. To explain this, let us give a description of CP2 in the spirit of the one we just gave of S4 . Let H(3, C) = {H ∈ M (3, C) | H = H ∗ } ¯ t is the adjoint be the space of complex 3 × 3 Hermitian matrices, where H ∗ = H matrix of H, obtained by conjugating first each entry of H and then transposing the matrix. We equip H(3, C) with the Hermitian inner product: H1 , H2  =

1 trace (H1 H2 ) . 2

(4.1)

As a vector space, with this inner product, H(3, C) is the ordinary Euclidean space E9 . Consider the subset P (2) of H(3, C) defined by: P (2) = {H ∈ H(3, C) | H 2 = H, and trace(H) = 1}.

(4.2)

4.3 Lemma. The set P (2) is a manifold, diffeomorphic to CP2 . Moreover, if we endow P (2) with the metric defined by (4.1), then P (2) is isometric to CP2 equipped with the Fubini-Study metric (of constant holomorphic sectional curvature 4). We remark that it is possible to describe CPn in a similar way, but we restrict to n = 2 because this is all we need. Proof. We claim that if H is in P (2), then it is an orthogonal projection over a complex line. In fact, if H is in P (2), then it is diagonalizable by a unitary matrix and its eigenvalues are 0 or 1, because H 2 = H. Since the trace is 1, two eigenvalues must be 0 and the other is 1. Hence H is a surjection of C3 over a complex line, and this map has to be an orthogonal projection because H is Hermitian. Conversely, it is clear that each line L ∈ C3 determines a unique orthogonal projection of C3 , and this is given by a matrix in P (2). The diffeomorphism in Lemma 4.3 is achieved by the map that carries H into the corresponding line in C3 . To prove that this map gives a metric equivalence, we notice that the unitary group U (3) ∗ acts on H(3, C)  by: H → U HU , and P (2) is an orbit of this action, with isotropy U (2) × U (1) . Thus,   P (2) ∼ = U (3)/ U (2) × U (1) ∼ = CP2 , and the metric on P (2) is obviously U (3)-invariant. Hence the induced metric on CP2 is also U (3)-invariant, and this characterizes the Fubini-Study metric, up to scaling.
Now we notice that if LH is the complex line in C3 that corresponds to a point of H ∈ P (2) under the diffeomorphism of Lemma 4.3, and if we pick a non-zero point (z1 , z2 , z3 ) ∈ LH , then H is the point in CP2 with projective coordi¯ corresponds the line with projective coordinates nates [z1 , z2 , z3 ]. To the matrix H [¯ z1 , z¯2 , z¯3 ]. Therefore we have:

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¯ coincides with the 4.4 Lemma. The involution j∗ of P (2) defined by j ∗ (H) = H, j

z1 , z¯2 , z¯3 ]. involution j of CP2 given by complex conjugation, [z1 , z2 , z3 ] → [¯ Let us denote by j the complex conjugation on both P (2) or CP2 . It is easy to see that the SO(3)-action commutes with complex conjugation on P (2) and therefore descends to an SO(3)-action on the orbit space P (2)/j. Each principal orbit becomes a copy of the flag manifold F 3 (2, 1); the projective space RP 2 corresponds to the matrices in P (2) with real entries, so it is the fixed point set of j. The other exceptional orbit is the quadric Q ∼ = S2 , and complex conjugation on it corresponds to identifying opposite points, hence its quotient by j is another copy of RP 2 . Summarizing, the quotient P (2)/j admits an SO(3)-action with two exceptional fibres, both copies of RP 2 and the principal orbits are copies of F 3 (2, 1). The space of orbits of this action on P (2)/j is the interval and the two exceptional orbits correspond to the end-points of the interval. As noted by Atiyah-Witten in [22], this is enough to deduce the classical Arnold-Kuiper-Massey theorem (see [8, 10, 116, 159]), saying that CP2 modulo conjugation is the 4-sphere, using the classification of closed 4-manifolds that admit a cohomogeneity 1-action of SO(3). However we prefer, just as in [135, 17], to give a direct proof of this theorem. This is what we do in the following section.

V.5

The Arnold-Kuiper-Massey theorem.

The previous discussion motivates an equivariant version of the Arnold-KuiperMassey theorem In this section we give (following [135]) a proof of this theorem. We construct an explicit algebraic map Φ : CP2 → S4 which is equivariant with respect to the cohomogeneity 1 isometric actions of SO(3, R) on CP2 and S4 and induces a diffeomorphism CP2 /conjugation ∼ = S4 . This method for proving the Arnold-Kuiper-Massey theorem is exactly the same method used by Atiyah and Berndt in [17] and has the advantage of generalizing to other situations. In this way they managed to prove that not only has one CP2 /conjugation ∼ = S4 , but 7 2 S , where H is the quaternionic also one has the theorem of Arnold H2 /SO(2) ∼ = projective plane, and also C 2 /Sp(1) ∼ = S13 , where C 2 is now the projective Cayley plane; this last result of was not known before. Furthermore, their diffeomorphisms are all equivariant with respect to certain cohomogeneity 1 group actions. To begin with, we notice that complex conjugation j on CP2 has fixed points, so it is not at all obvious that the quotient CP2 /j is even a smooth manifold. This is carefully explained in [153], so we only sketch here a few ideas. Away from the fixed point set Π ∼ = RP 2 , the involution j is free, so the quotient is a smooth manifold. The problem is on Π. A tubular neighborhood of Π in CP2 can be regarded as a normal open disc bundle, and conjugation carries each normal fibre into itself. Since the quotient of each normal 2-disc by the involution is again a 2-disc, it follows that the quotient CP2 /j is a topological manifold. Making this argument more carefully one gets that CP2 /j is in fact a P L-manifold, as noticed in [116], and therefore it is smooth, since every piecewise linear 4-manifold is

V.5. The Arnold-Kuiper-Massey theorem.

151

smooth. In [153] Marin defines the smooth structure on CP2 /j directly, without using P L-structures. An important point is that the smooth structure on CP2 /j is such that the obvious projection CP2 → CP2 /j is differentiable. Let us denote by Γ the aforementioned isometric action of SO(3, R) on S4 , and  the standard action of SO(3, R) on CP2 , which is by isometries with denote by Γ respect to the Fubini-Study metric. This action is defined either by considering SO(3, R) as a subgroup of O(3, C), acting on the space of lines in C3 , or via the action of SO(3, R) on the space of matrices P (2) ⊂ H(3, C) given by (O, A) → Ot AO. By Lemma 4.3, both metrics on CP2 are equivalent; also for every O ∈ SO(3, R), H ∈ P (2) and v ∈ C3 such that H(v) = v, one has: Ot HO(O−1 (v)) = O−1 (v), because O−1 = Ot . Hence both actions on CP2 ∼ = P (2) are equivalent. Similarly,  on CP2 and Γ on S4 , we say that these actions are given the SO(3, R)-action Γ equivariant if there exists a map Φ : CP2 → S4 , making commutative the following diagram:  Γ

SO(3, R) × CP2 −−−−→ CP2 ⏐ ⏐ ⏐ ⏐ Id×Φ Φ Γ

SO(3, R) × S4 −−−−→ S4  The map Φ carries In this case we say that Φ conjugates the actions Γ and Γ. orbits into orbits, i.e., the decompositions of CP2 and S4 into orbits are (smoothly) equivalent. Let us denote by Id3 the 3 × 3 identity matrix and by A = traceA2 the norm of a real symmetric matrix. The real part of a Hermitian matrix H is denoted Re(H). 5.1 Theorem. Identify S4 with the real 3 × 3 symmetric matrices A with trace(A) = trace(A2 ) = 0, and identify CP2 with the complex 3×3 Hermitian matrices H with trace 1 and H 2 = H. Then the map Φ : CP2 → S4 , Φ(H) =

1 3  13

Id3 − Re(H) Id3 − Re(H)

is a well-defined 2-to-1 branched covering, ramified over a copy of RP 2 embedded in S4 . This map is SO(3, R)-equivariant and also invariant by the complex conjugation j, inducing an equivariant diffeomorphism CP2 /j ∼ = S4 . We notice that Theorem 5.1, together with [106], imply that the image of RP 2 ⊂ CP2 under the above map is the image of RP 2 by the classical Veronese embedding (CP2 , RP 2 )
→ (CP5 , S4 ). The proof of Theorem 5.1 follows from several lemmas below.

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5.2 Lemma. Let A be a real (3 × 3)-matrix. Then A is the real part of a matrix H in P (2) if and only if it satisfies: (i) A is symmetric with trace 1; (ii) A has 0 as an eigenvalue and the other two eigenvalues λi and λj are roots of an equation of the form: λ2 − λ + k = 0 , for some constant k ∈ R with 0 ≤ k ≤ 14 . If A and H are as above, and if O ∈ SO(3, R) is such that Ot AO is a diagonal t matrix, then the imaginary part B of H, taken into √ its canonical form O BO, has only two possible non-zero entries, which are ± k. In particular, if k = 0, then H = A. Proof. Let us consider a matrix H ∈ P (2) and we decompose it in its real and ¯t ¯ t = At − i B t . Also H = H imaginary parts: H = A + i B . Then one has: H t t because H is Hermitian. Hence A = A and B = −B , i.e., A is symmetric and B is anti-symmetric. Thus the trace of A is 1, proving statement (i). One also has: H 2 = A2 − B 2 + i(AB + BA) , and H 2 = H because H is in P (2). Therefore: A = A2 −B 2 and, B = AB +BA . Now, A is symmetric, and so is A2 ; these two matrices obviously commute, so they can be diagonalized simultaneously by a matrix O ∈ SO(3, R). Since B 2 = A2 − A, one knows that Ot B 2 O is also diagonal: ⎞ ⎛ 0 µ1 0 Ot B 2 O = ⎝ 0 µ2 0 ⎠ , 0 0 µ3 with µi = λ2i − λi , for each i = 1, 2, 3, where the λi are the eigenvalues of A. But B is antisymmetric and commutes with B 2 , which is symmetric. Hence the same matrix O takes B to its canonical form: ⎛ ⎞ 0 a c Ot BO = ⎝−a 0 b⎠ , −c −b 0 for some a, b, c ∈ C. This implies:

⎛

−a2 − c2 t 2 t t ⎝ −bc O B O = (O BO)(O BO) = ab

−bc −a2 − b2 −ac

⎞ ab −ac ⎠ , −b2 − c2

which we know is a diagonal matrix. Therefore two of the numbers a, b, c must be zero. Assume for instance that a and b are 0, then both eigenvalues λ1 and λ3 are roots of the polynomial: λ2 − λ + c2 = 0 .

V.5. The Arnold-Kuiper-Massey theorem.

153

This implies: λ1 + λ3 = 1 , and λ1 · λ3 = c2 ≥ 0 . Hence λ2 = 0 (because the trace of A is 1), so 0 is an eigenvalue of A. The other eigenvalues λ1 and λ3 must be both ≥ 0 and ≤ 1, because their product is a nonnegative number and their sum is 1. Moreover the roots must be real, therefore k = c2 ≤ 14 , proving statement (ii). Also, in this case the eigenvalues of A determine the imaginary part B of H up to sign: ⎛ ⎞ 0 0 c B = ±O ⎝ 0 0 0⎠ Ot , −c 0 0 with c2 = λ1 − λ23 = λ3 − λ23 , proving in this case the last statement of Lemma 5.2. The other cases, when either {a, c} or {b, c} are both zero, are similar to the previous one. This proves that if A = Re(H) for some matrix H ∈ P (2), then A is as stated in Lemma 5.2. Conversely, given A satisfying these conditions, the arguments above tell us how to construct B so that these matrices are the real and imaginary parts of some H in P (2).
Now, given H ∈ P (2), its real part is Re(H) =

1 2

¯ Define (H + H).

ψ : P (2) → M (3, R), the space M (3, R) being the space of real (3 × 3)-matrices, by the formula: ψ(H) =

1 Id3 − Re(H) ∈ M (3, R) , 3

(5.3)

where Id3 is the (3 × 3)-identity matrix. In other words, ψ(H) is the real part of the matrix ( 13 Id3 − H). Since H ∈ P (2), it follows that ψ(H) is actually contained in S. It is clear that the above action of SO(3, R) on P (2) given by conjugation is equivalent, via the above diffeomorphism P (2) ∼ = CP2 , with the standard action studied above. It is also clear that, for every O ∈ SO(3, R), one has:    1 1 1 1 ¯ O = Ot ψ(H)O . ¯ ψ(Ot HO) = Id3 − Ot (H + H)O Id3 − (H + H) = Ot 3 2 3 2 Hence we have: 5.4 Lemma. The map ψ is equivariant. That is, for every O ∈ SO(3, R) and H ∈ P (2) one has: ψ(Ot HO) = Ot ψ(H)O . 5.5 Lemma. Given S ∈ S − {0}, there exists a unique positive t ∈ R, such that the matrix ( 13 Id3 − tS) is the real part of some matrix H ∈ P (2).

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Chapter V. Geometry and Topology of Quadrics

Proof. By Lemma 5.4, we can assume S is diagonal. Hence the matrix St = ( 13 Id3 − tS) is also diagonal, say ⎞ ⎛ 0 0 λ1 (t) 0 ⎠ λ2 (t) St = ⎝ 0 0 0 λ3 (t) with λi (t) = 13 − tµi , where the µi are the eigenvalues of S. We notice that for all t ∈ R, one has trace St = 1 − t (trace S) = 1 , because S has trace 0. Hence all these matrices satisfy condition (i) of Lemma 5.2. Let us look for the possible values of t that give solutions of Lemma 5.2. That is, we want t > 0 for which one eigenvalue λi (t) is 0 and the others satisfy that their sum is 1 and their product is ≥ 0 and ≤ 14 . Let us number the eigenvalues of S so that µ1 ≤ µ2 ≤ µ3 . Since their sum is 0 and S is not the zero matrix, one must have µ1 < 0 and µ3 > 0. If we want t as above, we must have that one λi (t) must vanish. Let us look for solutions with λ1 (t) = 0. This means t = 3µ1 1 < 0, and we want t > 0. Hence, there are no solutions with λ1 (t) = 0. Now let us look for solutions with λ2 (t) = 0. This implies t = 3µ1 2 ; for this to be possible we must have µ2 = 0. If µ2 < 0, then t < 0 and we want t to be positive. Thus, we only care about µ2 > 0. We have: λ1 (t) =

µ1 µ3 1 1 (1 − ) and λ3 (t) = (1 − ). 3 µ2 3 µ2

We have µ1 < 0 < µ2 , so λ1 (t) > 0. If µ2 < µ3 , then λ3 (t) < 0, thus the product λ1 (t)λ3 (t) is < 0, so there are no solutions like this that satisfy Lemma 5.5. The other possibility is µ2 = µ3 ; this implies λ3 (t) = 0 too. In this case one has λ1 (t) = 1 and λ2 (t) = λ3 (t) = 0, and t = 3µ1 2 is positive. Hence we have a solution, and this is unique because µ2 = µ3 . If µ2 = 0, then λ2 (t) cannot be 0 and we cannot find solutions like this. Summarizing, so far we have seen that: (i) there are no solutions satisfying Lemma 5.5 for which λ1 (t) = 0; (ii) if µ2 ≤ 0, there are no such solutions for which λ2 (t) = 0; and (iii) if µ2 = µ3 , then there is a unique solution satisfying Lemma 5.5, for which one has λ2 (t) = λ3 (t) = 0 and λ1 (t) = 1. Finally, let us look for solutions with λ3 (t) = 0, i.e., with t = 3µ1 3 . We know, by hypothesis, that µ2 ≤ µ3 and µ3 > 0. If µ2 = µ3 , then we are in the previous case and there is a unique positive t giving a solution satisfying Lemma 5.5. Let us assume now that µ2 < µ3 . Then we have:     1 1 µ1 µ2 λ1 (t) = and λ2 (t) = , 1− 1− 3 µ3 3 µ3 which are both ≥ 0. Since their sum is 1, it follows that each λi (t) is also ≤ 1.

V.5. The Arnold-Kuiper-Massey theorem.

155

The product of λ1 (t) and λ2 (t) satisfies:     1 µ1 µ2 1 µ1 + µ2 µ1 µ2 0 ≤ λ1 (t) · λ2 (t) = + = 1− 2+ 2 9 µ3 µ3 9 µ3   1 1 µ1 µ2 = ≤ , 2+ 9 (µ1 + µ2 )2 4 1 1 2 1 µ2 since µ1 + µ2 + µ3 = 0 and (µ1µ+µ 2 ≤ 4 because the inequality 4 (a + b) ≥ ab is 2) valid for any pair of real numbers a and b (and equality occurs iff a = b). Hence t = 3µ1 3 is the unique solution satisfying the conditions of Lemma 5.5.


We now “normalize” the map ψ so that its image is contained in S4 ⊂ S. For this we define a function 1

α(H) = [trace (ψ(H)2 )]− 2 , i.e., α(H) is the inverse of the norm of ψ(H) in S, and we set: Φ(H) = α(H) ψ(H) . One has:

!

trace [ψ(H)2 ] = trace # = trace

2 " 1 1 ¯ Id3 − (H + H) 3 2

$ 1 1 ¯ + 1 (H 2 + H ¯ 2 + HH ¯ + HH) ¯ Id3 − (H + H) 9 3 4

1 1 ¯ + HH), ¯ + trace (H H 6 4 ¯ + HH) ¯ which is always positive since the matrix (H H is positive semi-definite, so its trace is ≥ 0. Hence the maps α and Φ are well defined. It is clear that the image of Φ is contained in S4 ⊂ S, because the linearity of the trace implies: =

[trace (Φ(H))]2 = α2 (H) [trace ψ(H)]2 = 1 . It is also clear that Φ is SO(3, R)-equivariant, since the trace is invariant under conjugation and ψ is equivariant by Lemma 5.4. These considerations imply both Lemma 5.5 and the following lemma. 5.6 Lemma. The map Φ is an equivariant surjection from P (2) over S4 ⊂ S, and it is two-to-one, except over the image of the real matrices in P (2) where it is one-to-one. This gives the map in Theorem 5.1 that determines an equivariant diffeomorphism between S4 and CP2 modulo the involution given by conjugation. ¯ proving Then Φ is invariant under this involution, since Re(H) = Re(H), Theorem 5.1.
To complete the proof of Theorem 5.1 we notice that Φ is invariant under the involution of P (2) that corresponds to complex conjugation in CP2 (see Lemma 4.4 above).

Chapter VI

Real Singularities and Complex Geometry The topology of isolated complex singularities has long been studied by many authors, as we know already, and there is a beautiful and well-developed theory in this respect, though there are still many things to be understood. The previous chapters are a sample of this. This chapter is a turning point for us, as from now on we are concerned with the real counterpart of this theory, largely inspired by [168]. The theme here is the interplay between complex geometry and real analytic singularities. We consider several classes of real hypersurface and complete intersection singularities that arise naturally from complex geometry. The motivation for this is the pioneering work of S. L´ opez de Medrano, L. Meersseman and A. Verjovsky on new constructions of complex manifolds via holomorphic dynamics. These are now called LVM-manifolds. The first step they take is to construct real analytic complete intersection singularities associated to appropriate actions of Cm on Cn , n > 2m. We briefly describe this in Sections 1 and 2 below. In Section 3 we give a general method for constructing real analytic singularities using holomorphic vector fields in general, which is motivated by the constructions in Sections 1 and 2. In Section 4 we look at a very simple construction of real singularities, motivated by Section 3 and closely related to the material in Chapter VII of this text: the real hypersurfaces that arise when we consider a C-valued holomorphic function H on an open set U in Cn and we compose it with the projection onto a real line through the origin in C. As a byproduct of this we get explicit real analytic embeddings in R3 of closed oriented 2-manifolds of all genera.

VI.1

The space of Siegel leaves of a linear flow

This section is based on [50, 149, 150], which are our basic references. Let us consider now a holomorphic vector field in Cn of the form F (z) = (λ1 z1 , . . . , λn zn ),

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where the λi are non-zero complex numbers. The origin 0 ∈ Cn is the only zero (or singularity) of this vector field. The solutions of F , other than 0, are Riemann surfaces (non-compact), immersed in Cn . These are given by {z = eA T w}, with T ∈ C, w ∈ Cn and A the diagonal matrix with (λ1 , · · · , λn ) in its diagonal. The solutions of F give a 1-dimensional holomorphic foliation F with singular set 0 ∈ Cn . We want to look at the set of points in Cn − 0 where the foliation F has contacts with the codimension 1 real foliation S given by all the spheres in Cn with centre at 0. That is, we want to look at the points where these two foliations are tangent. Given z ∈ Cn − 0, we denote by Lz the leaf of F that passes through z. It is clear that Lz is tangent at z to the sphere S(0, z), with centre at 0 and radius the norm of z, iff the complex line spanned by the vector F (z) is contained in the tangent space Tz (S(0, z)); this is equivalent to saying that the Hermitian product of F (z) and z = (z1 , . . . , zn ) vanishes, i.e., F (z), z =

n 

λi |zi |2 = 0 .

i=1

In other words, the set of contacts of the two foliations F and S on Cn − 0 is the real analytic variety M ∗ = M − 0 defined by the intersection of the two quadrics:  % n % n & '  & '   n 2 n 2 λi |zi | λi |zi | = 0 ∩ z ∈ C | Im = 0 . VF = z ∈ C | Re i=1

i=1

There are two drastically different cases according as the convex hull, H(λ1 , . . . , λn ) ⊂ C , of the eigenvalues contains or does not contain the origin. To see why this fact makes such a big difference, observe that the analytic set VF , being the set of contacts of the two foliations, does not depend on the actual vector field F but only on the holomorphic foliation F that it defines. If we multiply F by a unit complex number eiθ , the spectrum of F is rotated by an angle θ, but the foliation remains unchanged. Hence, if 0 ∈ / H(λ1 , . . . , λn ) then, multiplying F by an appropriate unit complex number if necessary, we can assume that all the eigenvalues have positive real part. This implies that the equation: & % n  λi |zi |2 = 0 , Re i=1

does not have non-trivial solutions and therefore VF consists of 0 alone. This means that every leaf of F in Cn − 0 is transversal to all the spheres around 0. We will see that in the other case, when 0 ∈ H(λ1 , . . . , λn ), the variety VF has very rich and interesting geometry and topology.

VI.1. The space of Siegel leaves of a linear flow

159

1.1 Definition. If 0 ∈ H(λ1 , . . . , λn ) we say that the linear vector field F is in the Siegel domain. Otherwise we say that F is in the Poincar´e domain. The analogous situation in the real case, which we can draw, would be a sink (or a source) and a saddle point (see Figure 11).

VF

Figure 11: In the Poincar´e domain

in the Siegel domain.

The topology of the foliation F was studied in [50]. The case of linear flows in the Poincar´e domain is very interesting from the viewpoint of holomorphic dynamics, but it is not relevant for this text, whose purpose is to study the topology and geometry of analytic spaces, and for these vector fields the analytic space we are looking at consists of the origin alone. So we will restrict from now on to linear vector fields in the Siegel domain. It is noted in [50] that the equality, and even real dependence, of two eigenvalues of F complicates the topology of F and VF very much. Therefore one usually assumes the following generic hyperbolicity hypothesis: any two eigenvalues are independent over R: / Rλj , i = j ⇒ λi ∈

i, j = 1, . . . , n ,

(∗)

So we now let F be a linear vector field in the Siegel domain satisfying (*). It is clear that a  point z ∈ Cn − 0 is in VF iff the restriction of the real function n 2 2 d(z) = z = i=1 |zi | to the leaf Lz through z has a critical point at z. Furthermore, as noted in [50, §3], the fact that the solutions of F are parametrized by exponential maps implies that the leaves of F are concave. Thus, if a leaf L meets VF∗ , then it has a unique point in VF and it is the point in L of minimal distance to 0 ∈ Cn . Such a leaf is called a Siegel leaf. It is a copy of C embedded in Cn and can be characterized by its unique point in VF . Furthermore, the fact

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Chapter VI. Real Singularities and Complex Geometry

that the intersection L ∩ VF of each leaf that meets VF is at a local minimal point in L, implies that L ∩ VF is a transverse intersection. By the flow-box theorem for complex differential equations, this means that we have at each z ∈ VF∗ a neighborhood of the form Uz × D2 , where Uz is a disc of real dimension 2n − 2 and the second factor denotes small discs in the leaves. It follows that VF∗ is a smooth real submanifold of Cn of codimension 2. That is, VF is a real analytic complete intersection in Cn and the union W = VF∗ × C of all the Siegel leaves of F is an open subset of Cn that can be identified with the total space of the normal bundle of VF∗ . It is shown in [50] that W is actually dense in Cn . Notice that the n coordinate axes of Cn are contained in the complement of W : it is also shown in [50] that the complement Cn −W consists of the leaves of F that contain 0 in their closure; these are called Poincar´e leaves. It is an exercise to see that VF is globally embedded as a cone with vertex at 0 and base the intersection M = VF ∩ S2n−1 with the unit sphere, which is the link of the corresponding singularity. We summarize part of the above discussion in the following theorem, which re-phrases results in [50]: 1.2 Theorem. Let F (z) = (λ1 z1 , . . . , λn zn ) be a linear vector field in the Siegel domain, satisfying the hyperbolicity hypothesis. Then the real analytic variety:  VF =

% z ∈ C | Re n

n  i=1

& λi |zi |

2

' = 0

 ∩

z ∈ C | Im n

% n 

& λi |zi |

2

' = 0

,

i=1

is a complete intersection of real codimension 2 with an isolated singularity at 0, and the regular points of VF parametrize the Siegel leaves of F . So now the natural problem from the viewpoint of singularities is to determine the topology of the link M . This question was first addressed in general, and beautifully answered, by S. L´ opez de Medrano in [149, 150], though there are already a few remarks in this respect in [50]. This is closely related to a problem studied by C.T.C. Wall [260] in relation to the topological stability of smooth mappings. Let us describe briefly some of the results in [149, 150]. We begin by considering (as in [50]) the open set of unordered n-tuples of points Λ = {λ1 , . . . , λn } in C∗ satisfying the hyperbolicity hypothesis. This set has a connected component which is the Poincar´e domain, i.e., when 0 is not in the convex hull H(λ); the other components form the Siegel domain, and this is the one we are now interested in. It is clear that the topology of M = VF ∩ S2n−1 , being a compact smooth manifold, does not change within each component of the Siegel domain. It is pointed out in [50, p. 9] that the Siegel domain is connected for n = 3, 4 and it has three components for n = 5. This means that for n = 3, 4 the diffeomorphism type of the link is constant and for n = 5 there are at most three different diffeomorphism types. This was the starting point in [149]. Writing

VI.1. The space of Siegel leaves of a linear flow

161

λj = αj + iβj we see that the equations that determine VF are: 

α1 (x21 + y12 ) + · · · + αn (x2n + yn2 ) = 0 , β1 (x21 + y12 ) + · · · + βn (x2n + yn2 ) = 0 ,

(1.3)

and one can consider, more generally, the variety V given by the equations: 

a1 x21 + · · · + an x2n = 0 , b1 x21 + · · · + bn x2n = 0 ,

(1.4)

where the Ai = (ai , bi ) are pairs of real numbers. One would like to know what are the “degenerate” cases and what is the topology of the link in the “nondegenerate Obviously the first thing to look at is the case of a single  cases”. quadric ai x2i = 0, where the answer is classical: non-degenerate means no ai is zero, and the topology of the link is then given by the Morse index (or signature), i.e., by the number q of negative ai and the number p of positive ai . The link is then a product Sp−1 × Sq−1 . Let us look now at the case given by (1.4). We consider (following [149]) the points Ai = (ai , bi ) ∈ R2 . The natural non-degeneracy condition is that the gradient vector fields of these two equations be linearly independent at each point of V , i.e., that V is a complete intersection of codimension 2. It is easy to see that this condition is equivalent to saying that the origin is not in the convex hull of any pair Ai , Aj . This is called in [149] the weak hyperbolicity condition (w.h.c. for short), a notation taken from work by M. Chaperon. Under this assumption, the link M is a smooth submanifold of Rn of codimension 3. To describe the topology of the link, consider the configurations of n points Ai = (ai , bi ) as an unordered set of n vectors in R2 , some of which may coincide. If one configuration satisfying the w.h.c. is deformed into another without breaking this condition, then the corresponding links are diffeomorphic. Thus one needs to describe the connected components of the space of configurations satisfying the w.h.c., and in each such component one may deform the Ai to take them into a “normal form”. It is noted that the points Ai come in “bunches” that can be collapsed into one multiple point without breaking the w.h.c., and this multiple point can be pushed into the unit circle. Furthermore, one can move these points in the unit circle, preserving the w.h.c., so that they become the vertices of a regular polygon (see [150]), and a standard graph theory argument shows that this polygon has an odd number of vertices. Thus one can associate to each system of equations as in (1.4) and satisfying the w.h.c. a normal form determined by k = 2r + 1 vertices of a regular k-gon, each having a certain multiplicity ni (the number of points Ai concentrated at this vertex), and n = n1 + n2 + · · · + nk . This gives an odd cyclic partition of the positive integer n, and one may define two such partitions to be equivalent iff they differ by a rotation of the circle. Then one has ([150, Thm. 1]):

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Chapter VI. Real Singularities and Complex Geometry

1.5 Theorem. This association establishes a one-to-one correspondence between the set of connected components of the space of configurations of points in R2 satisfying the w.h.c. and the set of odd cyclic partitions of n. Thus the partition n = n1 + n2 + · · · + nk plays for the system of equations (1.4) the analogous role as the Morse signature n = p+q of a single non-degenerate quadric; the diffeomorphism types of the corresponding link of the singularity are given in terms of this partition [150, Thm. 2]: 1.6 Theorem. Let M be the link of the singularity determined by the partition n = n1 + n2 + · · · + nk , k = 2r + 1. Then (up to diffeomorphism): (i) If k = 1, then M = ∅. (ii) If k = 3, then M is a product of spheres: M ∼ = Sn1 −1 × Sn2 −1 × Sn3 −1 . (iii) For k ≥ 5, M is “always” a connected sum of products of spheres. More precisely, for each ni set di = ni + ni+1 + · · · + ni+r−1 , one has: if k = 5 and all di > 2, or if k > 5 and n = 7, then M is the connected sum: M ∼ = #ki=1 (Sdi −1 × Sn−di −2 ) . The original systems of equations as in (1.3), corresponding to linear vector fields in the Siegel domain with the hyperbolicity condition, correspond to systems as in (1.4) with n and all ni even. In particular, for vector fields in C3 the link M is always the 3-torus S1 × S1 × S1 . One has in general [149, Theorems 1 & 2]: 1.7 Theorem. The components of the Siegel domain are in one-to-one correspondence with the odd cyclic partitions n = n1 + · · · + nk of n into k > 1 positive integers, and the link M of the corresponding singularity is: (i) If k = 3, then M is a product of spheres: M ∼ = S2n1 −1 × S2n2 −1 × S2n3 −1 . (ii) If k = 2r + 1 > 3, then M ∼ = #ki=1 (S2di −1 × S2n−2di −2 ) . An important and difficult question is to determine how the topology of the singularities defined by equation (1.3) varies as we pass from one component in the Siegel domain to another one, i.e., as we break the weak hyperbolicity condition. This is beautifully answered, in a more general setting, in the recent article of F. Bosio and L. Meersseman [32]. This is a “wall-crossing” problem as they explain, and they show that crossing a wall means performing a precise surgery, which they describe. From the viewpoint of singularities, what we do when crossing a wall is to put two of the above complete intersection singularities in a 1-parameter family of singularities which are all complete intersections, except that they bifurcate when crossing the wall.

VI.2. The LVM manifolds

VI.2

163

Real singularities and the L´ opez de Medrano-Verjovsky-Meersseman manifolds

We now follow [160, 161, 162, 32] and extend the previous constructions to sets of m commuting vector fields on Cn . This type of situations were previously envisaged by N. Kuiper in [116], but his interest was more in the dynamics rather than in the geometry involved. In [160, 161, 162, 32], and also in [151], their interest is focused essentially on constructing and studying complex manifolds which arise as quotients of the links of the singularities that we describe here. Let m and n be positive integers such that n > 2m. Let Λ = (Λ1 , . . . , Λn ) be an n-tuple of vectors in Cm , Λi = (λ1i , . . . , λm i ) for i = 1, . . . , n, and let H(Λ1 , . . . , Λn ) be the convex hull of Λ in Cm . 2.1 Definition. The n-tuple Λ = (Λ1 , . . . , Λn ) is an admissible configuration if it satisfies: (i) the Siegel condition: 0 ∈ H(Λ); and (ii) the weak hyperbolicity condition (w.h.c.): for every 2m-tuple of integers / H(Λi1 , . . . , Λi2m ). (i1 , . . . , i2m ), 1 ≤ i1 < · · · < i2m ≤ n, we have 0 ∈ Geometrically this means that 0 is in the convex polytope H(Λ), but 0 is not contained in any hyperplane passing through 2m of its vertices. Note that conditions (i) and (ii) together form an open condition, so a small perturbation of an admissible configuration is again admissible. To an admissible configuration (Λ1 , . . . , Λn ) we associate the holomorphic foliation F of Cn generated by the m commuting vector fields (1 ≤ j ≤ m) Fj : (z1 , . . . , zn ) ∈ Cn −→

n  i=1

λji zi

∂ ; ∂zi

in other words, each vector field Fj has components (λj1 z1 , . . . , λjn zn ). The fact that these linear vector fields commute implies that they define together a holomorphic action of Cm on Cn and the orbits of this action are the leaves of F . This foliation is singular at the origin 0. In analogy with the previous case of m = 1, one has: 2.2 Definition. Let L be a leaf of F . If 0 is the closure of L, we say this is a Poincar´e leaf. Otherwise L is a Siegel leaf. Let us consider the Siegel leaves of F and consider the set VΛ of contacts of this foliation with the spheres around the origin. As in the previous case, this is defined by the real analytic equations:  ' n

 n 2 VΛ = z ∈ C Λi |zi | = 0 = VF1 ∩ · · · ∩ VFm , i=1

where each VFj is the variety defined in the previous section, i.e., VFj − 0 is the space of Siegel leaves of the vector field Fj . It is an exercise to show that the weak

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Chapter VI. Real Singularities and Complex Geometry

hyperbolicity condition implies that each VFj is a real codimension 2 complete intersection in Cn , and they all meet transversally away from 0; the variety VΛ is a global complete intersection in Cn of real codimension 2m and singular only at 0. (This is proved in [160, I.1] and it is analogous to Hamm’s result in [96] giving necessary and sufficient conditions for a set of Brieskorn varieties to define a complete intersection, c.f. II.7 above.) Again, the fact that the vector fields are linear implies that we can parametrize the leaves of F by exponential maps, and this implies that each Siegel leaf has a unique point in VΛ − 0, which is the point in the leaf of minimal distance to the origin. One has: 2.3 Theorem. Let (Λ1 , . . . , Λn ) be an admissible configuration as above, let F be the associated holomorphic foliation of Cn and let VΛ be the real analytic variety n n defined by VΛ = { z ∈ C | i=1 Λi |zi |2 = 0 } . Then: (i) VΛ is a complete intersection in Cn of dimension 2n − 2m, and it is singular only at 0. (ii) The smooth manifold VΛ∗ = VΛ − 0 parametrizes the Siegel leaves of F : each such leaf has a unique point in VΛ∗ ; this is the point in the leaf of minimal distance to the origin. (iii) The foliation F is everywhere transversal to VΛ∗ and the leaves of F can be identified with the fibres of the normal bundle of VΛ∗ . Hence VΛ∗ is, canonically, a complex manifold of dimension n − m. The last statement in this theorem, claiming that VΛ∗ has a canonical complex structure comes from the observation of A. Haefliger in [94], that if a smooth real submanifold N of a complex manifold X is everywhere transversal to a holomorphic foliation of complementary dimension, then the holomorphic foliated atlas of the foliation determines a holomorphic atlas for N . This does not mean however that VΛ∗ is embedded as a complex submanifold of Cn . Neither does this mean that the germ of VΛ at 0 is complex analytic, which is false in general. For instance, if m = 1 and n = 3, then we know from the previous section that the link is the 3-torus, and we know from [246] (see Chapter III above) that the 3-torus is not the link of an isolated surface singularity. It would be interesting to know whether or not these singularities are ever complex analytic, and I believe they are not (c.f. Chapter VII below). The results of [32] show that the topology of these singularities can be rather complicated and interesting, as is already shown in the case m = 1 of the previous section. These manifolds admit a natural action of the real torus Tn = (S1 )n given by: ((u1 , . . . , un ), (z1 , . . . , zn )) −→ (u1 z1 , . . . , un zn ) ∈ Cn , for (u1 , . . . , un ) ∈ Tn and (z1 , . . . , zn ) ∈ Cn . This action leaves invariant the link MF and they show in [32] (in analogy with [149, 150] for m = 1) that this action can be used to associate a natural convex polytope to MF /Tn , which captures the topology of the link.

VI.2. The LVM manifolds

165

The principal goal of [151, 160, 124] was to use the previous constructions to obtain new families of compact, complex non-symplectic manifolds. These are now called the L´ opez de Medrano-Verjovsky-Meersseman (LVM for short) manifolds. A motivation for this comes from the generalized Hopf manifolds of [46, 94]. Recall that the Hopf manifolds are constructed as follows: given a real number r > 1, we can take in Cn − 0 the action of Z given by m · z = rm z. The quotient is a manifold diffeomorphic to S2n−1 × S1 . Since the action is holomorphic and totally discontinuous, the quotient inherits a natural complex structure. This construction was generalized by Haefliger taking quotients of Cn − 0 by other holomorphic and totally discontinuous actions of Z, obtaining complex manifolds diffeomorphic to S2n−1 × S1 but with different complex structures. For this he used the observation just mentioned above, that a smooth manifold transversal to a holomorphic foliation inherits a complex structure. The construction in [46] of compact manifolds that generalizes those of Hopf is given by considering the Brieskorn singularities V(a0 ,...,an ) = {z0a0 + · · · + znan = 0} , and taking the quotient of V ∗ = V(a0 ,...,an ) − 0 by the analogous Z-action: given a real number r > 1, define a Z-action on V ∗ by m · (z1 , . . . , zn ) = (rm/a0 z0 , . . . , rm/an zn ) . The quotient V ∗ /Z is a compact complex manifold diffeomorphic to M ×S1 , where M is the link. Even when the link is the standard sphere, some of these complex structures are different to those obtained in [94]. Another motivation for the LVM-manifolds comes from [49, 268]. Calabi and Eckmann [49] generalized Hopf’s construction to give complex structures on all products of spheres S2n−1 × S2m−1 , n, m > 1. Each of these manifolds is the total space of a principal bundle S2n−1 × S2m−1 → CPn−1 × CPm−1 , with fibre the 2-torus; endowing the torus with the structure of an elliptic curve one turns S2n−1 × S2m−1 into a complex manifold. The Calabi-Eckmann manifolds were generalized by Loeb-Nicolau in [268], inspired in Haefliger’s work. They consider a linear vector field F in Cn+m in the Poincar´e domain, satisfying a weak hyperbolicity hypothesis (weaker than the one in §1 above). Then they prove that there is an embedding of S2n−1 × S2m−1 in Cn+m which is transversal to the foliation F of F , and therefore (by [94]) inherits a complex structure from that of F . In this way they get a large class of complex structures on these products of spheres, including those of [49]. The LVM-manifolds combine all these ideas to give a new construction of compact complex manifolds. This class contains “most” of the manifolds constructed in [94, 268], and many more, where “most” means that in both cases

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the LVM-manifolds leave out certain complex structures that arise by considering vector fields with a certain type of resonances which are valid in [94, 268] but not in [151, 160, 124]. To construct the LVM-manifolds we start with an admissible configuration Λ = (Λ1 , . . . , Λn ) as above, let F be the associated holomorphic foliation of Cn and let VΛ be the real analytic variety defined by VΛ = { z ∈ Cn |

n 

Λi |zi |2 = 0 } ,

i=1

so that VΛ∗ = VΛ − 0 is the space of Siegel leaves of the Cm -action. Let S ⊂ Cn be the union of all the Siegel leaves, so that S is VΛ∗ saturated by the Cm -action. Observe that the usual C∗ -action: t · (z1 , . . . , zn ) → (tz1 , . . . , tzn ) commutes with the Cm -action and both actions have S as an invariant set. Therefore they give together a free action of Cm × C∗ on S by holomorphic transformations. The quotient  N = NΛ = S (Cm × C∗ ) isa complex manifold. To prove that it is compact we observe that the quotient S Cm is the manifold VΛ∗ , hence N is nothing but the projectivization of VΛ∗ . This can also be regarded as the quotient of the link M of 0 in VΛ divided by the S1 action induced from that of C∗ . We also observe that one can follow the method of [46] described above and construct Z-actions on VΛ∗ so that the orbit space is a compact, complex manifold which is a principal bundle over N with fibre an elliptic curve. The manifold N is, by definition, an LVM-manifold. These manifolds have fascinating geometry and topology. Their topology is beautifully described in [32]. In [160] it is shown (using [143]) that the m vector fields defined by the admissible configuration Λ give rise to m holomorphic vector fields on N which define a (non-singular) holomorphic foliation G of dimension m (recall N has complex dimension (n−m−1) ≥ m). And if the configuration Λ satisfies a certain “rational condition (K)”, then [162, Theorem A] this  foliation is by compact, complex tori. Furthermore, in this case the quotient N G is a projective (quasi-regular)  toric variety equipped with an orbifold structure. The projection N → N G is a generalized Calabi-Eckmann fibration, in the notation of [162] Corollary C, and they also show that every LVM manifold has an arbitrary small deformation of its complex structure so that the deformed manifold is the total space of a generalized Calabi-Eckmann fibration. Conversely, Theorem G and Corollary H in [162] tell us that every projective toric manifold, and every projective quasi-regular toric variety, can be realized as a quotient N G, for an appropriate LVM-manifold N .

VI.3. Real singularities and holomorphic vector fields

VI.3

167

Real singularities and holomorphic vector fields

Let us now give a method for constructing codimension 2 real analytic complete intersection singularities in Cn with a very rich geometry and topology; this is motivated by the constructions in Section 1 and is reminiscent of the geometry of complex singularities. This construction, taken from [226], is actually inspired by [88], where the goal is to study the topology of holomorphic vector fields near an isolated singularity, in a similar way to the one used by Milnor to study the topology of an analytic space near an isolated singularity. It would be interesting to make a similar study for singularities of higher codimension, using ideas of §2 above. Let χ(Cn , 0) denote the space of all germs of continuous vector fields at 0 ∈ Cn , and let F, X be elements in χ(Cn , 0). One has a continuous map, ψF,X : Cn ∼ = R2 , = R2n → C ∼ defined by ψF,X (z) = F (z), X(z), where F (z), X(z) =

n 

Fi (z) · X i (z)

i=1

is the usual Hermitian product. We note that if F and X are both differentiable of class C r , then ψF,X is of class C r ; if F and X are both real analytic, then ψF,X is also real analytic, but even if both F and X are complex analytic, ψF,X is not complex analytic, unless X is constant. 3.1 Examples. (i) Let F be a linear vector field in Cn , F (z) = (λ1 z1 , . . . , λn zn ), and let X be the radial vector field X(z) = (z1 , . . . , zn ). Then ψF,X (z) =

n 

λi |zi |2

i=1 −1 is the map envisaged in §1 and its zero-set VF,X = ψF,X (0) consists of the origin union the set of contacts of the foliation F of F with the foliation given by all the spheres around 0. Thus, if F is in the Poincar´e domain, then VF,X = {0}, and if F is in the Siegel domain and satisfies the weak hyperbolicity condition, then VF,X − 0 is the space of Siegel leaves of F . p q f (z) (ii) Let f : C2 → C be the Pham-Brieskorn polynomial  = z1 + z2 , with  ∂f ∂f p, q > 2, let F be the Hamiltonian vector field F = ∂z2 , − ∂z1 , and let X = (a, b) be constant. Then ∂f ∂f −b ψF,X = a ∂z2 ∂z1

and VF,X is the polar curve of f and a linear form (much studied by B. Teissier, Lˆe D˜ ung Tr´ ang and others).

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(iii) Let F be holomorphic and let X be the gradient vector field of a real analytic function f : Cn → R. Then VF,X is the union of the zeroes of F , the zeroes of X and the polar variety of the foliations F (defined by F ) and the one given by the level surfaces of f , i.e., the set of points in Cn where the two foliations are tangent (c.f. [251]). If f is the square of the function distance to the origin, then this is example i). Although some of the things we say below hold in general for F and X continuous vector fields, we restrict ourselves to the case of this last example, F holomorphic and X the gradient vector field of a real analytic function f : Cn → R, where the geometry is very rich and interesting. −1 (0) be the polar variety of F and the level surfaces of f , Let VF,X = ψF,X that we denote by S. So VF,X is defined by the real analytic equations: Re F (z), X(z) = 0

and

Im F (z), X(z) = 0 .

Away from VF,X these two foliations meet transversally, defining a foliation Γ by real curves which are tangent to both foliations. In order to study the geometry of VF,X it is convenient to modify the function that determines it by composing ψF,X with the automorphism ι : C → C given by ι(z) = i z¯. The composition ψF,X = ι ◦ ψ is the map z → i F (z), X(z) . Clearly −1 one has VF,X = ψF,X (0). 3.2 Lemma. The foliation Γ is given by the integrals of the real analytic vector field τ (z) = ψF,X (z) · F (z) , whose zero locus is VF,X . Proof. It is clear that τ (z) is always tangent to F , because at each point z ∈ Cn , τ (z) is F (z) multiplied by a complex number. To prove that τ (z) is tangent to S we must prove that τ (z) is normal to X(z). One has: τ (z), X(z) =  (i F (z), X(z) ) · F (z), X(z)  = (i F (z), X(z) ) ·  F (z), X(z)  = i F (z), X(z)2. Hence Re τ (z), X(z) = 0 , so τ (z) is normal to X(z) because the real part of the
Hermitian product is the usual inner product in R2n ∼ = Cn . This vector field τ describes the behavior of F in the direction determined by the level surfaces of f . For instance, if X(z) = z and F is a linear vector field in the Poincar´e domain with generic eigenvalues, then one knows by work of Guckenheimer that τ is Morse-Smale, a fact used in [50] to prove that the vector field F is structurally stable. The following immediate consequence of Lemma 3.2 gives a geometric interpretation of the map ψF,X . For simplicity set ψ = ψF,X .

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169

3.3 Proposition. For each z ∈ Cn − VF,X , the argument of the complex number  ψ(z) is the angle by which we must rotate the vector F (z) in its complex line (counter-clock-wise) to make it tangent to the level surface of f at z. Let us now look at the map φ = φF,X : Cn − VF,X → S1 ⊂ C given by  i F (z), X(z) ψ(z) φ  z −→ arg(ψ(z)) = =  |F (z), X(z)| |ψ(z)| ψθ

and set Eθ = φ−1 (eiθ ). For each θ ∈ [0, π) ⊂ R we define a map Cn −→ R by ψθ (z) = Re eiθ F (z), X(z) , and we set Vθ = ψθ−1 (0). One has the following decomposition theorem, which is reminiscent of Milnor’s fibration theorem, but so far we are not claiming anything in this respect. We will return to that point in Chapter VII. 3.4 Theorem. The union of all Vθ covers Cn ; their intersection is the polar variety VF,X and each pair of antipodal fibres Eθ and Eθ+π are naturally glued along VF,X forming the variety Vθ . That is, for θ ∈ [0, π) one has: Cn = ∪Vθ ;

VF,X = ∩ ∪ Vθ ;

and Mθ = Eθ ∪ VF,X ∪ Eθ+π .

The proof of this result is exactly the same as the proof of 4.1 below. We now recall that X is the gradient ∇f of a function f . By restricting f to the leaves of F one gets a vector field XF whose solutions are contained in the leaves of F ; locally, for z ∈ / VF,X , XF (z) is obtained by projecting X(z) to the complex line spanned by F . The zeroes (or singularities) of XF form precisely the variety VF,X . We say (following Thom [251]) that a point z ∈ VF,X is a generic contact of the two foliations if none of them is singular at z and the corresponding zero of XF is non-degenerate. This means that the restriction of f to the leaf has a Morse singularity at z. Hence these contacts are either non-degenerate local minimal points in the leaves of F (with respect to the level surfaces of f ), or local maximal points, or saddle points. Just as in the case of the varieties of Siegel leaves in Sections 1 and 2, one has that in either case of a generic contact, each leaf of F , which is (real) 2-dimensional, is locally contributing to VF,X with one point. It follows that at a generic contact the polar variety VF,X is smooth of real codimension 2. Furthermore, with a little more work one can prove that the converse also holds and one has the following theorem (we refer to [88, 2.3] for details). 3.5 Theorem. Assume that the function f and the vector field F vanish only at 0. Then the variety of contacts VF,X is a real analytic geometric complete intersection in Cn of real codimension 2 and smooth away from 0 if and only if all the contacts of F and the level surfaces of f are generic. Of course in all this discussion we can consider F and X to be defined only on an open set U of Cn and not on the whole space.

170

VI.4

Chapter VI. Real Singularities and Complex Geometry

On the topology of certain real hypersurface singularities

This section is taken from [228] and it is motivated by [168, 197] and by the previous section. Consider now a connected open set D ⊂ Cn+1 , 0 ∈ D, and let H : (D, 0) → (C, 0) be a continuous function. For each real line Lθ ⊂ C passing through the origin with an angle θ, θ ∈ [0, π) , we let πθ : C → Lθ be the orthogonal projection; set hθ = πθ ◦ H , so that h0 and h π2 are, respectively, the real and the imaginary parts −1 of H. We set Mθ = h−1 (0). We define the map φ˜ : D − V → S1 θ (0) and V = H iH(z) iθ 1 ˜ by φ(z) = ||H(z)|| . For each e ∈ S , we set Eθ = φ˜−1 (eiθ ). The following lemma establishes that one has in general a decomposition similar to the one in Theorem 3.4 above: 4.1 Lemma. One has: D = ∪Mθ and V = ∩Mθ , for θ ∈ [0, π). Also, for each θ ∈ [0, π) one has: Mθ = Eθ ∪ V ∪ Eθ+π . Similarly, if Sε is a sphere embedded in D with centre at 0, one has, Sε = ∪(Mθ ∩ Sε )

V ∩ Sε = ∩(Mθ ∩ Sε ) ;

and

(M ∩ Sε ) = (Eθ ∩ Sε ) ∪ (V ∩ Sε ) ∪ (Eθ+π ∩ Sε ) . Proof. If z ∈ Mθ , then H(z) is contained in the line passing through 0 with an angle θ ± π2 . Hence z ∈ Mθ1 ∩ Mθ2 if and only if z ∈ V , where θ1 = θ2 + kπ , k ∈ Z. We prove that V = ∩Mθ for each θ ∈ [0, π). It is clear that each point in D is either in V itself or else it is in a certain Eθ . So the claim D = ∪Mθ follows from the claim Mθ = Eθ ∪ V ∪ Eθ+π . We prove this last claim. For this we notice that Mθ is the set of points z ∈ Cn+1 such that eiθ H(z) is a purely imaginary number. One has: Mθ = {z ∈ Cn+1 | Re eiθ H(z) = 0} . It is now clear that V ⊂ Mθ . Let us prove that Eθ ⊂ Mθ . If z ∈ Eθ , then iH(z) = eiθ , ||H(z)||

e−iθ iH(z) = 1, ||H(z)||

which implies

hence Re

eiθ H(z) = 0. ||H(z)||

Thus one has z ∈ Mθ . Similarly, if z ∈ Eθ+π , then iH(z) = ei(θ+π) = −eiθ , ||H(z)||

so

e−iθ

iH(z) = −1 , ||H(z)||

VI.4. Topology of real hypersurface singularities

171

which implies Re e−iθ H(z) = 0. Hence one has Eθ+π ⊂ Mθ . Conversely, if z ∈ Mθ , then Re e−iθ H(z) = 0 . If H(z) = 0, one has z ∈ V and there is nothing to prove. If H(z) = 0, then iH(z) = ±1 , e−iθ ||H(z)|| so z is in Eθ or in Eθ+π , according to its sign, and we arrive at the formula above. To prove the second statement in the lemma we just restrict the previous
discussion to the sphere Sε . 4.2 Theorem. Assume now that H is holomorpic; let hθ = πθ ◦ H be as before and let H : Sε − K → S1 φ= ||H|| be the usual Milnor fibration, where K = V ∩ Sε is the link of 0. Then: (i) Each Mθ = h−1 θ (0) is a real analytic hypersurface in D, whose singular set is the singular set of V . (ii) Each pair of antipodal fibres Fθ and Fθ+π of φ are naturally glued together along K, forming a real analytic variety isomorphic to Nθ = Mθ ∩ Sε . Proof. The second statement in Theorem 4.2 is a consequence of the lemma above, because the map φ˜ is φ followed by the diffeomorphism z → iz of C. Hence the intersection of each Mθ with each sphere Sε is a real analytic variety of dimension 2n; since Mθ is a cone near 0, by [168], we know that Mθ is a real hypersurface. Its singular set consists of the critical points of hθ , i.e., the points where all the partial derivatives of hθ vanish. This set does not change if we multiply hθ by the number e−iθ . Thus, to prove the claim about the singularities of Mθ , it is enough to consider the case θ = 0, i.e., for the real part of H, f = Re H. One has, f=

1 (H + H) , 2

where H is the complex conjugate of H. Therefore, 2∆f =

 ∂H ∂H ∂H  ∂H , , ,..., , ∂z1 ∂z1 ∂zn+1 ∂zn+1

because the partial derivatives of H with respect to the z i are all 0, the partial ∂H derivatives of H with respect to the zi are all 0 and ∂z = ∂H ∂zi . Thus, the critical i points of H are the critical points of f .
This theorem tells us that the link Nθ of Mθ is, in some sense, the double of the Milnor fibre of H, but Nθ is singular if K is singular. We recall (see Ch. I.6 above) that even in this case there is a Milnor fibration H : H −1 (S1δ ) ∩ Bε → S1δ ,

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Chapter VI. Real Singularities and Complex Geometry

where S1δ is a small circle centred at 0 ∈ C and Bε is a small open ball in Cn+1 . The closure of each fibre Eθ = H −1 (eiθ ) in this fibration is a compact non-singular  while the closure of the Milnor fibre F is the union of variety with boundary K, F with the link K, which may be singular. I thank Lˆe D˜ ung Tr´ ang for explaining  onto K; this can be to me that there is a natural contraction function from K constructed by lifting to Cn+1 a radial vector field in C around 0. It would be interesting to study the relation of this function to our construction (c.f. [163]). Equivalently, one may study the relation between the double of the Milnor fibre, which is a closed manifold, and the link Nθ of the real analytic variety Mθ = h−1 θ (0). We notice that M0 and M π2 are, respectively, the sets of points where Re H(z) = 0 and Im H(z) = 0. If the singular set of V consists of an isolated point, one has the following theorem: 4.3 Theorem. If 0 ∈ D is an isolated critical point of H, then the Mθ  s are nonsingular away from 0, and they are all homeomorphic. The link Nθ = Mθ ∩ Sε is diffeomorphic to the double of the Milnor fibre F of H, hence Nθ is (n − 1)connected; Nθ is always stably parallelizable, and it is actually parallelizable if and only if n is odd and the Milnor number µ of H is 1. We recall that a manifold M is parallelizable if its tangent bundle T M is trivial; M is stably parallelizable if T M ⊕ (k) is trivial, where (k) is a trivial bundle over M , or equivalently, if M can be embedded in some sphere SN with trivial normal bundle. Since the topology of the Milnor fibre of H and the link K are well understood, these theorems determine the topology of the real hypersurfaces Mθ . Proof. The claims that the Mθ are regular away from 0, that they are all homeomorphic (actually diffeomorphic away from 0) and the link is diffeomorphic to the double of the Milnor fibre of f , are all consequences of Theorem 4.2. That Nθ is connected follows because F is connected, by [168]. Furthermore, if n > 1, then K is connected and F is simply connected, by [168]. Hence Van Kampen’s Theorem [239, p. 151] implies that Nθ is simply connected. Moreover, by [168], F is a wedge of n-spheres and K is (n − 2)-connected, thus Mayer-Vietoris (reduced if n=2) implies that one has H1 (Nθ ; Z) ∼ = ··· ∼ = Hn−1 (Nθ ; Z) ∼ = 0. Therefore Hurewicz’s isomorphism [239, p. 397] implies that Nθ is (n − 1)connected. Finally, Nθ is stably parallelizable because it is a codimension 1, oriented submanifold of Sε . Thus, [112, Th. IX], Nθ is parallelizable if and only if its Euler-Poincar´e characteristic χ(Nθ ) vanishes. One has: χ(Nθ ) = 2χ(F ) = 2 + 2(−1)n µ ,

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173

where µ > 0 is the Milnor number of H, since Nθ is the double of F , and F is a
wedge of µ n-spheres. So χ(Nθ ) = 0 if and only if n is odd and µ = 1. The following corollary is actually a special case of the theorem above, but we think it deserves to be stated on its own. 4.4 Corollary. Let f : (Cn+1 , 0) → (C, 0) be a holomorphic map with an isolated critical point at 0 ∈ Cn+1 and let F be the Milnor fibre of f , so F = f −1 (t) ∩ Bε for some small ball Bε and some t with |t| sufficiently small. Let M0 be the link of the real part of f , i.e., of the function Re f : (Cn+1 , 0) → (R, 0). Then M0 is diffeomorphic to the double of F . 4.5 Example. Consider again the polynomial f (z1 , z2 ) = z1p + z2q , with p, q > 1. The topology of this singularity is well understood (see Ch. I). Its link K is a torus link (or knot if p, q are relatively prime) and its Milnor number µ is (p − 1)(q − 1); we recall that the Milnor fibre F is in this case an oriented surface in the 3-sphere, with boundary K and the homotopy type of µ circles. Now consider the real and the imaginary parts of f , f1 = Re f = z1p + z2q + z¯1p + z¯2q , f2 = Im f = z1p + z2q − z¯1p − z¯2q . Both define real hypersurfaces in C2 , which are cones over their links L1 , L2 . By the theorem above, L1 and L2 are homeomorphic, actually diffeomorphic, and they are the double of the Milnor fibre F . Hence L1 and L2 are closed, oriented surfaces of genus (p − 1)(q − 1) in the 3-sphere S3 ⊂ C2 . This can be interesting because it provides explicit analytic embeddings of surfaces of all genera in S3 ⊂ C2 . For instance, if p = 2 = q, then K is the Hopf link, F is a cylinder S1 × I and L1 , L2 are tori S1 × S1 , obtained by taking two copies of F and gluing them along their boundary. The link K is the intersection of the tori L1 , L2 .

Chapter VII

Real Singularities with a Milnor Fibration A map f : (Rn+k , 0) → (Rk , 0) , n, k > 0 , satisfies the Milnor condition at 0 if it is a submersion at every point in a punctured neighborhood of 0 ∈ Rn+k . Milnor’s fibration theorem says that every such map determines a locally trivial fibre bundle φ Sε − M → Sk−1 for every sufficiently small sphere Sε ⊂ Rn centred at 0, where M is the link. However this projection φ in general may not be given by the obvious map |ff | . The goal of this chapter is to construct real analytic singularities for which Milnor’s condition holds and one actually has that the projection map is f |f | , just as it is for complex singularities. In this case we say that the singularity satisfies the strong Milnor condition. The results in this chapter are based on [226, 227, 214]. For completeness we present also related results of [111, 218] where the authors discuss the difference between the Milnor condition and the strong Milnor condition, and they give sufficient conditions for a map f as above to satisfy the strong Milnor condition. In particular, for weighted homogeneous maps both conditions are equivalent. The constructions in this chapter are inspired by those in Chapter VI and the singularities we get are reminiscent of the classical Pham-Brieskorn singularities, so we call them twisted Pham-Brieskorn singularities.

VII.1

Milnor’s fibration theorem revisited

Let us consider real analytic functions f : (Rn , 0) → (Rp , 0), n ≥ p. 1.1 Definition. We say that the map f satisfies the Milnor condition at 0 if the derivative Df (x) has rank p at every point x ∈ U − 0, where U is an open neighborhood of 0 ∈ Rn , i.e., if f is a local submersion at every point in a punctured neighborhood of 0 ∈ Rn .

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Chapter VII. Real Singularities with a Milnor Fibration

One has the following theorem of Milnor, that we proved in Chapter I: 1.2 Theorem. If f satisfies the condition of Definition 1.1, then for every ε > 0 sufficiently small, and for every δ > 0 sufficiently small with respect to ε, one has that f |f −1 (Cδ )∩Bε : f −1 (Cδ ) ∩ Bε −→ Cδ , is a fibre bundle, where Cδ ⊂ C is the circle in C of radius δ and centre at 0, and Bε is the closed ball in Rn of radius ε and centre 0. Of course every complex-valued holomorphic function with an isolated critical point in its domain satisfies these conditions, and so does if we compose such function with a real analytic diffeomorphism of the target C. The interesting point here is to find examples which are honestly real analytic. Milnor exhibited the following examples in his book, suggested to him by N. Kuiper, of functions satisfying the condition of Definition 1.1, and therefore giving rise to fibrations. Let A denote either the complex numbers, the quaternions or the Cayley numbers, and define h:A×A→A×R, by h(x, y) = (2x¯ y , |y|2 − |x|2 ). Milnor first proves ([168, 11.6]) that this mapping carries the unit sphere of A × A to the unit sphere of A × R by a Hopf fibration (see [242, p. 109]). Then he defines, more generally, f : An × An → A × R , by f (x, y) = (2x, y, y2 − x2 ) , where ·, · is the Hermitian inner product in A. One has that this map is a local submersion on a punctured neighborhood of (0, 0) ∈ An × An . The link M of the corresponding singularity is the Stiefel manifold of 2-frames in An and the Milnor fibre in this case is a disc bundle over the unit sphere of An . Milnor asked a number of questions regarding his theorem for real singularities. As Milnor pointed out in his book, the hypothesis of Df having maximal rank everywhere near 0 is too strong and it is difficult to find examples, even for p = 2. There are in fact pairs (n, p) as above for which no such examples exist, see [53, 208]. For p = 2, which is the most relevant case for this work, what is generic is to have real curves in R2 converging to (0, 0), whose inverse image consists of points where the Jacobian matrix has rank less than 2. Consider for instance the maps ψ : Cn ∼ = R2 of Chapter VI: = R2n → C ∼ ψ(z) = F (z), z =

n 

λi |zi |2 ,

i=1

where F = (λ1 z1 , . . . , λn zn ) is a linear vector field. We know from the previous chapter that if F is in the Siegel domain and satisfies the weak hyperbolicity

VII.2. The strong Milnor condition

177

condition, then the zeroes of ψ define a codimension 2 real complete intersection in R2n with an isolated singularity at 0, and it has a very rich geometry and topology. These maps are reminiscent of Milnor’s examples above and could be natural examples for maps satisfying the Milnor condition at 0. However it is an exercise to see that, regardless of what the eigenvalues of F are, the derivative of these maps has rank less than 2 on the n axis of Cn . Milnor actually asked whether there exist “non-trivial” examples satisfying the condition of Definition 1.1 when p = 2. This question was answered positively by Looijenga [145] for n even and by Church and Lamotke [53] for n odd, using Looijenga’s technique. However, no explicit examples of such singularities are given in those articles. Roughly speaking, their proof consists in showing that on one hand, if Df has maximal rank everywhere near 0, then the pair (Sε , M ) is a Neuwirth-Stallings pair (c.f. [188, 240]), where M = Sε ∩ f −1 (0) is the link of the singularity; and conversely, a Neuwirth-Stallings pair with appropriate conditions determines an isolated singularity as above. Then they use an inductive process to show that for p = 2 one can always construct such a Neuwirth-Stallings pair, proving the existence of non-trivial examples for all n > 1. The first explicit non-trivial example of a real analytic singularity with target R2 satisfying the Milnor condition at 0, other than those of Milnor, was given by A’Campo [2]. This is given by the map Cm+2 → C defined by 2 (u, v, z1 , . . . , zm ) −→ uv(¯ u + v¯) + z12 + · · · + zm ,

which is not holomorphic due to the presence of complex conjugation. In §3 below we construct, following [227, 214], infinite families of singularities satisfying Milnor’s condition, which are in the same vein as that of A’Campo.

VII.2

The strong Milnor condition

The problem of studying real singularities satisfying the Milnor condition at 0, for which the map |ff | extends to all of Sε − M → Sp−1 as the projection of a fibre bundle (as in the case of holomorphic maps), was first studied in [111], and this turns out to be quite subtle. When p = 2 such singularities define open book decompositions on the spheres with the link M as binding; M is a fibred knot (or maybe a link), with the Milnor fibre f −1 (0) ∩ Dε being a Seifert surface for M. We recall (Theorem 1.2 above) that given an analytic map f : Rn → R2 satisfying the Milnor condition at 0, for each small circle Cδ ⊂ R2 around the origin, the restriction of f to f −1 (Cδ ) ∩ Bε , where Bε is a small closed disc around 0 ∈ Rn , is a fibre bundle over the circle with projection map f . Then using a vector field we may inflate the tube f −1 (Cδ ) ∩ Bε to make it become the complement (of a tubular neighborhood) of the link M in the sphere Sn−1 and in this way we have ε 1 ∼ − M fibreing over C . The point is that when we inflate the tube to S Sn−1 = δ ε carry it into the sphere we lose control, in general, of what map we end up having on the sphere. It is also hard to keep control of the behavior of the fibres as we

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get closer to the link. When n is even and f is holomorphic, then Milnor showed that you can keep  control of this process, so that you end up having the map f as projection (or f |f | if we want the image to be the unit circle S1 ⊂ C). 2.1 Definition. Let f = (f1 , f2 ) : (Rn , 0) → (R2 , 0) be analytic and satisfy the Milnor condition at 0. Let M be the link; f satisfies the strong Milnor condition at 0 if for every sufficiently small sphere Sε around 0, the map f : Sε − M → S1 |f | is the projection of a fibre bundle. The strong Milnor condition is not always satisfied by maps that satisfy the Milnor condition, as noticed by Milnor in [168, p. 99]. Jacquemard gave two conditions that were sufficient to guarantee that |ff | actually extends to all of Sε − M as the projection map of a fibre bundle. The first condition (A) is geometric: that the angle between the gradient vector fields of these two functions be bounded; the second condition (B) is algebraic: 2.2 Theorem. (Jacquemard) Let f : (Rn , 0) → (R2 , 0) be an analytic map-germ. If the component functions f1 and f2 of f satisfy the following two conditions, then f satisfies the strong Milnor condition. These conditions are: A) there exists a neighborhood U of the origin in Rn and a real number 0 < ρ < 1 such that for all x ∈ U − 0 one has: |grad f1 (x), grad f2 (x)| ≤ 1 − ρ, || grad f1 (x)|| · || grad f2 (x)|| where ·, · is the usual inner product in Rn ; and, B) if εn denotes the local ring of analytic map-germs at the origin in Rn , then the integral closures in εn of the ideals generated by the partial derivatives  ∂f

1

∂x1

,

∂f1 ∂f1  ,..., ∂x2 ∂xn

and

 ∂f

2

∂x1

,

∂f2 ∂f2  ,..., ∂x2 ∂xn

(2.3)

coincide. Again, no explicit examples of such maps were given in [111] (other than complex polynomials with an isolated critical point). It was noted in [214] that the above condition (B) can be relaxed and still have sufficient conditions to guarantee the strong Milnor condition. For this we recall the notion of the real integral closure of an ideal as given in [86]. 2.4 Definition. Let I be an ideal in the ring εm . The real integral closure of I, denoted by I R , is the set of h ∈ εm such that for all analytic ϕ : (R, 0) → (Rm , 0), we have h ◦ ϕ ∈ (ϕ∗ (I))ε1 .

VII.2. The strong Milnor condition

179

Given f : (Rn , 0) → (R2 , 0) as above, let us set: Condition BR : The real integral closures of the Jacobian ideals (2.3) coincide. For complex analytic germs both conditions (B) and (BR ) are equivalent (see [248], [86]). As pointed out in [214], essentially the same proof of Jacquemard in [111] gives: 2.5 Theorem. Let f : (Rn , 0) → (R2 , 0) be an analytic map-germ that satisfies the Milnor condition. If its components f1 , f2 satisfy the condition (A) of Theorem 2.2 and condition (BR ), then f satisfies the strong Milnor condition. This improvement of Theorem 2.2 was used in [214] to prove a stability theorem for real singularities with the strong Milnor condition (see §6 below). This was also used in [218] to find a theorem in the spirit of Theorem 2.5 but using “regularity conditions” instead of Jacquemard’s conditions. This is inspired in [226, 227] and related to VI.4 above. To explain this we need to introduce some concepts from [26, 27]. Let M be a smooth n-manifold, and let X, Y be submanifolds of M such that Y ⊂ X. 2.6 Definition. The pair (X, Y ) is (a)-regular at y0 ∈ Y if for each sequence of points {xi } → y0 such that the corresponding sequence of tangent spaces {Txi X} of X at xi converges to some T in the Grassmannian of (dim X)-planes in Rn , then one has Ty0 Y ⊂ T . We say that (X, Y ) is (a)-regular if it is (a)-regular at every y0 in Y. 2.7 Definition. Let ρ : M → R be a smooth non-negative function such that ρ−1 (0) = Y . The pair (X, Y ) is C-regular at y0 ∈ Y with respect to the control function ρ if for each sequence of points {xi } → y0 such that the sequence of planes {ker Dρ(xi ) ∩ Txi X} converges to a plane T in the Grassmannian of (dim X − 1)planes in Rn , then Ty0 Y ⊂ T . The pair (X, Y ) is C-regular with respect to the control function ρ if it is C-regular for every point y0 ∈ Y with respect to the function ρ. It is easy to see that C-regularity implies (a)-regularity. Now consider a map-germ f : (Rn , 0) → (R2 , 0) as above, satisfying the Milnor condition at 0. Just as in the previous chapter, let πθ : C → L−θ be the linear projection to the line L−θ forming angle −θ with the horizontal axis in C = R2 , and consider the family fθ = πθ ◦f . The projections πθ are all submersions, hence the {fθ } are all real-valued maps with an isolated critical point at 0. For each θ ∈ [0, π), set Vθ∗ = fθ−1 (0) − {0} ⊂ Rn . Now consider the map F : (Rn × [0, π)) → R×R defined by F (x, θ) =  fθ (x), and set Xf = F −1 (0)−(0×[0, π)), Yf = 0×[0, π). Thus Yf ⊂ X f and Xf = Vθ∗ for all θ ∈ [0, π). 2.8 Definition. The family {fθ } is C-regular at y0 ∈ Y with respect to a control function ρ if the pair (Xf , Yf ) is C-regular at y0 with respect to ρ.

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Chapter VII. Real Singularities with a Milnor Fibration

The main result of [218] is: 2.9 Theorem. Let f : (Rn , 0) → (R2 , 0) be a real analytic map-germ with an isolated critical point at the origin, such that f satisfies the Milnor condition at 0 and the → (R, 0) satisfies the C-regularity condition with respect to the family fθ : (Rn , 0)  function ρ(x, θ) = i x2i . Then, f satisfies the strong Milnor condition. Let us give an outline of their proof. First they notice (Lemma 2.6 in [214]) that one has a decomposition of the ambient space as given in VI.§4 above: there is a neighborhood U ⊂ Rn of 0 such that for every x ∈ U − {0}: (i) U = ∪θ (Vθ ∩ U ), 0 ≤ θ < π ; Vθ = fθ−1 (0). (ii) V = ∩θ Vθ = Vθ1 ∩ Vθ2 , where V = f −1 (0), θ1 = θ2 ; θ1 , θ2 ∈ [0, π). (iii) For each θ ∈ [0, π), Vθ = Eθ ∪ V ∪ Eθ+π , where Eα = φ−1 (eiα ) with φ : (x)  U − V → S1 , φ(x) = i |ff (x)| .

Furthermore, one also has: (iv) If f satisfies the Milnor condition at 0, then for each θ ∈ [0, π), Vθ∗ = Vθ − {0} is a real smooth submanifold of real codimension 1 of U − {0}, given by the union of Eθ , Eθ+ π2 and V − {0}. Now, by [26, 255], the C-regularity condition of the family fθ implies that there exists a family of germs of homeomorphisms hθ : (Rn , 0) → (Rn , 0), with h0 = Id and hθ (x) = x, ∀x ∈ fθ−1 (0) (preserving small spheres centred at the origin), such that fθ ◦ hθ (x) = f0 (x). Hence the hypersurfaces Vθ are homeomorphic. It then follows from [26] that there exists a sufficiently small ε > 0 such that each Vθ meets the sphere Sn−1 transversally for all θ. Each Eθ is a submanifold of ε = (Eθ ∩ Sn−1 ) ∪ (V ∩ Sn−1 ) ∪ (Eθ+ π2 ∩ Sn−1 ). One can thus get U , and Vθ ∩ Sn−1 ε ε ε ε n−1 satisfying properties similar to (i)–(iv) above. a decomposition of the sphere S Let Fθ := Eθ ∩ Sn−1 . It follows from (iii) that letting Mε = V ∩ Sε one has: ε Vθ ∩ Sn−1 = Fθ ∪ Mε ∪ Fθ+π . ε For each θ1 , θ2 one has (fθ1 ◦ hθ1 )−1 (0) = f0−1 (0) = (fθ2 ◦ hθ2 )−1 (0). Then, −1 −1 n−1 h−1 ) = Vθ2 ∩ Sn−1 . ε θ1 (Vθ1 ) = hθ2 (Vθ2 ). Hence hθ2 ◦ hθ1 (Vθ1 ∩ Sε Using this one can define an R-action on the sphere, Γα : R × Sn−1 → Sn−1 , ε ε −1 by x −→ hθ ◦ hα (x) for x ∈ Mα . Since hα (x) = x one has that Γα is well defined and for all α ∈ R and x ∈ Fα one has: −1 n−1 hθ ◦ h−1 ) = Eθ ∩ Sn−1 = Fθ . α (Fα ) = hθ ◦ hα (Eα ∩ Sε ε

Moreover, the action Γα is transversal to the fibres and satisfies Γα (Kε ) = Γα (M ∩ Sn−1 ) = Γα (M ) ∩ Sn−1 = M ∩ Sn−1 = Kε . ε ε ε Hence it leaves the link invariant. Now, proceeding as in [168] Theorem 4.8, we
get that |ff | is the projection map of a locally trivial fibre bundle.

VII.3. Real singularities of the Pham-Brieskorn type

181

As a corollary they obtain a generalization of Theorem 4.0.2 of [214]. For this we recall (c.f. III.1 above) that a polynomial h in m real variables is weighted homogeneous if there exist non-zero integers (q1 , . . . , qm ) and a positive integer d, such that: h(tq1 x1 , . . . , tqn xm ) = td h(x1 , . . . , xm ) . Equivalently, we demand that there exist non-zero rational numbers (w1 , . . . , wm ), called the weights of h, for which h is a sum of monomials z1i1 · · · znim such that i1 in + ···+ = 1. w1 wm A singular variety V (or a map-germ) is said to be quasi-homogeneous if it is defined by polynomials in m real variables which are weighted homogeneous of the same weights. 2.10 Corollary. Let f : (Rn , 0) → (R2 , 0) be a quasi-homogeneous polynomial mapgerm. Then f satisfies the Milnor condition at 0 iff it satisfies the strong Milnor condition at 0. This follows from Theorem 2.9 since it is well known that in this case the family fθ is C-regular (see [26]). 2.11 Remarks. (i) It is worth saying that the proof of Theorem 2.9 is similar in spirit to that of Theorem 4.2 below, which is more elementary since in that case the R-action that gives the local triviality of the fibres can be given explicitly, so one does not need to use Bekka’s regularity. This has the advantage that one gets for free the monodromy map of the corresponding Milnor fibrations. (ii) M.A. Ruas and R.N.A. Santos in [218, Theorems 4.4 and 4.8] prove that Theorem 2.9 is stronger than Theorem 2.5 and 2.2: if a map-germ satisfies Jacquemard’s conditions (A) and (B), or (BR ), then it satisfies C-regularity, but not conversely.

VII.3

Real singularities of the Pham-Brieskorn type

We recall that the Pham-Brieskorn singularities are defined by { z ∈ Cn+1 | z1a1 + · · · + znan = 0 ; ai ≥ 2 ∀ i = 1, . . . , n} , and one may consider more generally the variety defined by the complex polynomial f (z) = λ1 z1a1 + · · · + λn znan , where the λi are non-zero complex numbers, which is essentially the same. We have seen in Chapter I of this book, and in almost every other chapter, several remarkable geometric and topological features of these singularities. Let us consider

182

Chapter VII. Real Singularities with a Milnor Fibration

now the following real analytic analogues of them: ψ(z) = λ1 z1a1 z 1 + · · · + λn znan z n , or more generally, ψ(z) = λ1 z1a1 z σ1 + · · · + λn znan z σn , where σ = {σ1 , . . . , σn } is a permutation of the set {1, . . . , n}. 3.1 Definition. A twisted Pham-Brieskorn real singularity of class {a1 , . . . , an ; σ} is a singularity in Cn defined by a polynomial map ψ(z) = λ1 z1a1 z σ1 + · · · + λn znan z σn , where each ai ≥ 2, i = 1, . . . , n, the λi are non-zero complex numbers and σ = {σ1 , . . . , σn } is a permutation of the set {1, . . . , n}. We call weights the exponents {a1 , . . . , an }, and the permutation σ is the twisting. So for instance, if n = 2 then the maps in question are of two classes: λ1 z1a1 z 1 + λ2 z2a2 z 2

or

λ1 z1a1 z 2 + λ2 z2a2 z 1 .

For n = 3, one has three essentially different classes: λ1 z1a1 z 1 + λ2 z2a2 z 2 + λ3 z3a3 z 3

;

λ1 z1a1 z 2 + λ2 z2a2 z 1 + λ3 z3a3 z 3

or

λ1 z1a1 z 2 + λ2 z2a2 z 3 + λ3 z3a3 z 1 , and so on. In Section 5 below we prove that if the twisting σ is the identity, then the corresponding singularity is topologically equivalent to the Pham-Brieskorn singularity z1a1 −1 + · · · + znan −1 = 0 , where the ai are the weights. In this section we prove that for all twisted Pham-Brieskorn singularities, the corresponding variety V = ψ −1 (0) is a codimension 2 complete intersection in Cn with a unique singularity at 0, and the corresponding maps satisfy the Milnor condition at 0. In the following section we prove that they further satisfy the strong Milnor condition at 0. We observe that if we let F, X be holomorphic vector fields of the form: F = (k1 zσa11 , . . . , kn zσann ) , X = (t1 z1b1 , . . . , tn znbn ) , where the ki and the ti are non-zero complex numbers, the ai and bi are positive integers and {σ1 , . . . , σn } is a permutation of the set {1, . . . , n}, and if we let ψF,X (z) : Cn → C be the real analytic function defined (as in Chapter VI) by ψF,X (z) = F (z), X(z) =

n  i=1

ki · ti · zσaii · z bi i ,

VII.3. Real singularities of the Pham-Brieskorn type

183

then the twisted Pham-Brieskorn singularities correspond to the case when the exponents bi are all 1. The purpose of this section is to classify the vector fields F and X as above, for which the corresponding map ψF,X satisfies the Milnor condition at 0 ∈ Cn ∼ = R2n . This has the advantage of giving us more singularities that satisfy the Milnor condition, other than the twisted Pham-Brieskorn singularities. For this we think of ψ := ψF,X as a map into R2 and we let (ψ1 , ψ2 ) be its components, which correspond to the real and imaginary parts of ψ. We want to determine the rank of its derivative. Suppose first that F is of the form F (z) = (k1 z1a1 , . . . , kn znan ) and X = (z1b1 , . . . , znbn ) , i.e., the permutation σ is the identity. In this case one has: 1 1 ψ1 = (ki ziai z bi i + k i z ai i zibi ) , ψ2 = (ki ziai z bi i − k i z ai i zibi ) . 2 i=1 2 i=1 n

n

Let us consider the 2 × 2 minor of ψ corresponding to the partial derivatives of (ψ1 , ψ2 ) with respect to zi and z i . Its determinant (multiplied by −2) is: ||ki ||2 a2i ||zi ||2ai −2 ||zi ||2bi − ||ki ||2 b2i ||zi ||2ai ||zi ||2bi −2 , so this is 0 iff zi = 0 or ai = bi . Hence, if zi = 0 and ai = bi for all i, then ψF,X is a submersion. Conversely, if ai = bi for some i, then the rank of ψ(z) is less than 2 whenever we are on the zi -axis, proving that in this case the function ψF,X does not satisfy the Milnor condition at 0.  = Given F and X as above, let us set: F = (k1 t1 zσa11 , . . . , kn t¯n zσann ) , and X b1 bn ∗ (z1 , . . . , zn ) , then ki ti is in C for each i, and one has: ψF,X = ψF ,X . Thus we can assume, with no loss of generality, that each ti is 1. Now, given F = (k1 zσa11 , . . . , kn zσann ) , where σ is a permutation of {1, . . . , n}, we can split σ into cycles of length r ≥ 1. Consider such a cycle and re-label the a components so that the corresponding components of F are (k1 z2a1 , . . . , kr−1 zr r−1 , kr z1ar ). In this case ψ is of the form: ψ(z) = (k1 z2a1 z b11 + · · · + kr z1ar z brr ) + (terms in other variables). Hence, to determine whether or not ψ satisfies the Milnor condition at 0, we only need to look at the minors of the Jacobian matrix determined by the various cycles of σ. In other words, it is enough to consider the case when F is a vector field in Cr of the form F (z) = (k1 z2a1 , k2 z3a2 , . . . , kr z1ar ) , and X = (z1b1 , . . . , zrbr ). The case r = 1 was discussed above, so we assume r > 1. In this case the determinant of the minor of ψ corresponding to the partial derivatives of (ψ1 , ψ2 ) with respect to zi , z i , multiplied by −2, is: ||ki−1 ||2 a2i−1 ||zi ||2ai−1 −2 ||zi−1 ||2bi−1 − ||ki ||2 b2i ||zi+1 ||2ai ||zi ||2bi −2 .

184

Chapter VII. Real Singularities with a Milnor Fibration

If the rank of Dψ at a point z = (z1 , . . . , zr ) is less than 2, then all these determinants must vanish; making λi = ||ki ||2 and νi = ||zi ||2 for all i = 1, . . . , r, we get the system of equations: (3.2)

λ1 a21 ν2a1 −1 ν1b1 = λ2 b22 ν3a2 ν2b2 −1 , λ2 a22 ν3a2 −1 ν2b2 = λ3 b23 ν4a3 ν3b3 −1 , .. . λr a2r ν1ar −1 νrbr = λ1 b21 ν2a1 ν1b1 −1 .

Suppose now that each ai is greater than 1, and also that one bi is greater than 1, say bj > 1. Let us assume first that r ≥ 5. Considering the indices as integers modulo r, there are exactly three equations that involve νj+1 , these are: a

λi−1 a2i−1 νi i−1

−1 bi−1 νi−1

ai = λi b2i νi+1 νibi −1 ,

for i = j , j + 1 , j + 2 . It is then clear that the rank of Dψ is less than 2 at all points in Cr satisfying zj−1 = zj = zj+2 = zj+3 = 0 . Hence, if some bi is more than 1, then ψ is not a submersion on a punctured neighborhood of 0. The cases r = 2, 3, 4 can be easily worked out case by case, using the same technique as above. Conversely, we claim that ψ is in fact a submersion near 0 when bi = 1 for all i (in fact this statement also holds if each bi is either 0 or 1). This is proved in [227] and it is a straightforward computation using the above system of equations (3.2): first we note that if (ν1 , . . . , νr ) satisfies the system (3.2) and some νi = 0, then νi = 0 for all i, so the solution is trivial. Thus we can assume νi > 0 for all i. Multiplying the left-hand side and the right-hand side of all the equations in (3.2) we get: n n a a [λi a2i νi i−1 ] = [λi νi i−1 ] . i=1

i=1

This implies ai = 1 for all i, because λi and νi are both non-zero for all i, which is a contradiction. Thus we arrive at Theorem 3.3 below; we recall that F, X are vector fields of the form: F = (k1 zσa11 , . . . , kn zσann ) , X = (t1 z1b1 , . . . , tn znbn ) , where the ki and the ti are non-zero complex numbers, all ai and bi are positive integers, {σ1 , . . . , σn } is some permutation of the set {1, . . . , n} and ψF,X (z) = F (z), X(z) . 3.3 Theorem. Assume ai > 1 for all i = 1, . . . , n. Then ψF,X satisfies the Milnor condition at 0 ∈ Cn if and only if either one of the following two conditions is satisfied: (i) The permutation σ is the identity, so that F (z) = (k1 z1a1 , . . . , kn znan ), and one has ai = bi for each i = 1, . . . , n.

VII.3. Real singularities of the Pham-Brieskorn type

185

(ii) Split σ into cycles σ1 , . . . , σm , where each σj is a permutation of length oj . Then: for each j = 1, . . . , m such that oj = 1, one must have that the corresponding exponents satisfy ai = bi ; for each j = 1, . . . , m such that oj > 1, the exponents bi corresponding to these components are all 1. 3.4 Remark. We notice that in the cases above, whenever ψF,X is a submersion away from 0, one actually has that ψF,X is a submersion on all of Cn − {0}. This is not always the case. For instance, if F = (k1 z2a , k2 z1 ), a > 1, and X = (z1 , z2 ), then ψF,X is a submersion near 0 ∈ C2 , but the rank of Dψ drops at every point (0, z2 ) such that ||z2 ||a−1 = ||k2 ||/||k1 ||. As we will see, this is related to the fact that the singularities in Theorem 3.3 are all quasi-homogeneous. Let us motivate the following theorem with an example: 3.5 Example. Let F (z) = (k1 z2a1 , k2 z3a2 , k3 z4 , k4 z1 ) , a1 ≥ 1 , a2 ≥ 1 , and X = (z1 , z2 , z3 , z4 ). The system of equations (3.2) becomes λ1 a21 ν2a1 −1 ν1 = λ2 ν3a2 ; λ2 a22 ν3a2 −1 ν2 = λ3 ν4 ; λ3 ν3 = λ4 ν1 ; λ4 ν4 = λ1 ν2a1 , where νi = ||zi ||2 . If a2 = 1, then every z ∈ C4 of the form z = (0, z2 , 0, z4 ) , with ν4 =

λ2 λ2 λ4 ν2 , ν2a1 −1 = , λ3 λ1 λ3

is a point where ψF,X is not a submersion. If a1 = 1, this implies |k1 k3 | = |k2 k4 | or else z is the origin. 3.6 Theorem. We now let F be as above, but we allow the ai to be ≥ 1 , and X = (z1 , . . . , zn ) . Then the function ψF,X satisfies the Milnor condition at 0 iff it satisfies the following conditions: (i) For each permutation of σ of odd length, there is at least one ai strictly bigger than 1. (ii) For each permutation of σ of even length, either there is at least one ai strictly bigger than 1, or else the corresponding coefficients ki , . . . , ki+2m+1 satisfy: ||ki ki+2 . . . ki+2m || = ||ki+1 ki+3 . . . ki+2m+1 || . Proof. As in the proof of Theorem 3.3 above, we can assume σ has only one cycle. In this case the system of equations (3.2) becomes: (3.7)

λ1 a21 ν2a1 −1 ν1 = λ2 ν3a2 , λ2 a22 ν3a2 −1 ν2 = λ3 ν4a3 , .. . λn−1 a2n−1 νnan−1 −1 νn−1 = λn ν1an , λn a2n ν1an −1 νn = λ1 ν2a1 .

186

Chapter VII. Real Singularities with a Milnor Fibration

If n is odd, one has that if νi = 0 for some i, then νi = 0 for all i, so the solution is trivial. Hence we can assume νi is non-zero for all i. Then, as before, multiplying the left-hand side of these equations, and the right-hand side, we obtain: n n a a [λi a2i νi i−1 ] = [λi νi i−1 ] . i=1

i=1

Therefore ai = 1 for all i, or else νi = 0 for all i and we have the trivial solution. Hence, if we assume that at least one ai is greater than 1, then ψF,X is a submersion on Cn − 0. Conversely, if n = 2m + 1 and ai = 1 for all i, then one can easily verify that the system of equations (3.7) has non-trivial solutions given by: λ2m−1 λ2m−1 λ2m−3 . . . λ3 λ1 ν2m−1 = · · · = ν1 ; λ2m λ2m λ2m−2 . . . λ2 λ2m−2 λ2m−2 λ2m−4 . . . λ2 ν2m = ν2m−2 = · · · = ν2 ; λ2m−1 λ2m−1 λ2m−3 . . . λ3 λ2m+1 λ2m−1 . . . λ3 ν1 ; ν2 = λ2m λ2m−2 . . . λ2

ν2m+1 =

where ν1 = ||z1 ||2 can be taken as near to 0 as we please, proving statement (i) in the theorem. Assume now n = 2m is even and at least one ai > 1; for simplicity we assume a1 > 1. It is easy to see that if one νeven is 0 then νi is 0 for all i. However, if a νodd is zero, this only implies that all the νodd are 0. Hence, as in the example above, one has in this case that a point z ∈ Cn of the form z = (0, z1 , 0, z2 , . . . , 0, z2m ) is a solution of (3.7) iff it satisfies: ν2m =

λ1 a1 λ2m−1 a2m−1 λ3 a3 ν2 ; ν2m−2 = ν2m , . . . , ν2 = ν , λ2m λ2m−2 λ2 4

and a a3 ...a2m−1 −1

ν2 1

=

1 λ3 λ5 a3 λ7 a3 a5 λ2 ( λ4 ) ( λ6 )

a3 a5 ...an−1

. . . ( λλn1 )

.

Hence every non-trivial solution is at a bounded distance away from 0 ∈ Cn , thus ψF,X is a submersion on a punctured neighborhood of 0. Finally assume n = 2m and ai = 1 for all i. Then the system of equations (3.7) splits into the two independent systems of m-equations given by: λ1 ν1 = λ2 ν3 , λ3 ν3 = λ4 ν5 , . . . , and λ2 ν2 = λ3 ν4 , λ4 ν4 = λ5 ν6 , . . . ; in either case, the corresponding system of equations has a non-trivial solution iff: ||λi λi+2 . . . λi+2m || = ||λi+1 λi+3 . . . λi+2m+1 || , proving the theorem.




VII.4. Twisted Pham-Brieskorn singularities

VII.4

187

Twisted Pham-Brieskorn singularities and the strong Milnor condition

In this section we prove that all the twisted Pham-Brieskorn singularities satisfy the strong Milnor condition. We also prove that they are all quasi-homogeneous singularities (which is not obvious). Hence the fact that they satisfy the strong Milnor condition is a consequence of Corollary 2.10 and Theorem 3.3 above. However we prove this fact directly, using only Theorem 3.3 and not Corollary 2.10, because this is almost as hard as proving that they are quasi-homogeneous (which we must prove anyhow), and it throws light into the geometry of these singularities, since we obtain the monodromy map of the corresponding Milnor fibrations. More generally, in this section we classify the vector fields F and X as above, F = (k1 zσa11 , . . . , kn zσann )

and X = (t1 z1b1 , . . . , tn znbn ) ,

such that the function ψ := ψF,X satisfies the strong Milnor condition at 0. For each θ ∈ R, define a map ψθ : Cn → R by: 1  iθ (e Fi (z)X i (z) + e−iθ F i (z)Xi (z)) , 2 i=1 n

ψθ (z) = Reeiθ F (z), X(z) =

and set Vθ := ψθ−1 (0). It is clear that Vθ = Vθ+πr for all r ∈ Z. One also has ψθ (z) = Re eiθ ψ(z), so that ψθ is ψ followed by the projection from C onto the line L−θ that passes through the origin in C with an inclination of −θ radians. Thus ψθ is a surjection whenever ψ is a surjection. We recall that, just as in VI.4 and in the proof of 2.8, one has a decomposition of Cn in the Vθ satisfying: (i) Cn is the union of all Vθ , θ ∈ [0, π) . (ii) V = ∩θ∈[0,π) Vθ = Vθ1 ∩ Vθ2 , for any distinct θ1 θ2 ∈ [0, π) . (iii) For each θ ∈ [0, π) , one has Vθ = Eθ ∪ V ∪ Eθ+π , where Eα = φ−1 (eiα ) and φ : Cn − V → S1 is the function  = arg(iF (z), z) . φ(z) (iv) If ψ satisfies the Milnor condition at 0, then each Vθ∗ := Vθ − {0} is a smooth real submanifold of Cn of codimension 1. For every sufficiently small sphere Sε in Cn with centre at 0, we let Mε = V ∩ Sε be the link of V and we set φε = φ =

ψ : Sε2n−1 − Mε → S1 . |ψ|

(4.1)

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4.2 Theorem. Let F = (k1 zσa11 , . . . , kn zσann ) and X = (t1 z1b1 , . . . , tn znbn ) be vector fields as above, and assume the following conditions are satisfied: (i) for each cycle of σ of length 1, the corresponding exponents satisfy ai = bi ; (ii) for each cycle of length r > 1 , r odd, one has ai ≥ 1, bi = 1 and at least one ai is strictly bigger than 1, for each i = 1, . . . , r . (iii) for each cycle of σ of length r > 1 , r even, for each i = 1, . . . , r , one has ai ≥ 1, bi = 1 and at least two ai , say ai1 and ai2 , are strictly bigger than 1, with i1 being odd and i2 even in the cycle. Then ψF,X satisfies the strong Milnor condition at 0. That is, for every sufficiently small sphere Sε2n−1 , (4.1) is a C ∞ fibre bundle. Each pair of antipodal fibres φ−1 (eiθ ) , φ−1 (ei(θ+π) ), are glued together along Mε forming the smooth (2n − 1)manifold: Vθ ∩ Sε2n−1 = {z ∈ Sε2n−1 ⊂ Cn | Re eiθ F (z), z = 0} , and the monodromy of this bundle is the first return map of the S1 -action in Lemma 4.3 below. Notice that the twisted Pham-Brieskorn singularities satisfy the hypothesis of Theorem 4.2 and therefore the statements of this theorem hold for them. From now on we assume F, X are vector fields satisfying the conditions of Theorem 4.2, so we know already, by Section 3, that ψF,X satisfies the Milnor condition at 0. Theorem 4.2 will be a consequence of the previous results in §3 and the following two lemmas: 4.3 Lemma. There exists a norm-preserving, smooth action γλ of S1 on Cn , permuting the Vθ . More precisely, if we think of this action as an R-action, via the identification S1 ∼ = R mod (π), then for every λ ∈ R, γλ carries Vθ into V[θ+λ] , where [θ + λ] means the residue class of (θ + λ) modulo π. Hence, V is an invariant set for this action. This lemma is proved by constructing explicitly the S1 -action. Before we do this, let us observe that Lemma 4.3 implies that the Vθ∗ of Theorem 4.2 are all diffeomorphic and the map φ : Cn − V → S1 is the projection map of a locally  trivial C ∞ -fibre bundle. It is clear that φ(z) is φ followed by the diffeomorphism  of C given by z → iz. Therefore one has that φ : Cn − V → S1 is also a locally trivial fibre bundle. To prove Theorem 4.2 we shall prove that the restriction of φ to every sphere around 0 ∈ Cn , is also the projection map of a locally trivial fibre bundle (notice this is more than we need: it would be sufficient to prove this claim for small spheres). This is an immediate consequence of the previous discussion and the following lemma: 4.4 Lemma. With the above hypotheses, there exists a flow {ft } on Cn , whose orbits are transversal to every sphere around 0 ∈ Cn , except for 0 itself, which is a fixed point, and they converge to 0 ∈ Cn when the time tends to −∞. This flow leaves invariant each Vθ , thus it also leaves V invariant.

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189

Of course Lemma 4.4 implies that each Vθ is embedded (globally) as a cone in Cn , intersecting transversally each sphere around 0 ∈ Cn , and so does V . Before proving these lemmas, let us state a result which is a corollary of the proof of Lemma 4.4. 4.5 Corollary. If F and X satisfy the conditions of Theorem 4.2, then the polynomial in R2n ∼ = Cn defined by ψF,X , is weighted homogeneous. To prove Lemmas 4.3 and 4.4, let us split Cn into direct summands, according to the cycles of σ, and construct the corresponding flows on each of these direct summands. Proof of Lemma 4.4. We shall construct an action Γ : R+ × Cn → Cn of the form Γ(t,(z1 ,...,zn )) = (tm1 z1 ,...,tmn zn ), mi ∈ Q, mi > 0, such that ψ(Γ(t, z)) = tψ(z). It is clear that this implies Lemma 4.4 and Corollary 4.5. Consider first a 1-cycle (if there is any). This corresponds to a monomial in ψ := ψF,X of the form ki ziai z bi i . 1 Define w := Γ(t, zi ) = (t (ai +bi ) zi ). Then ki wiai w bi i = t(ki ziai z bi i ), as we wanted. Now consider an r-cycle, r > 1, and let ψ(r) be the polynomial consisting of the monomials in ψ containing the variables in this cycle. We re-label the a components so that this cycle is (k1 z2a1 , . . . , kr−1 zr r−1 , kr z1ar ). Let B be the r × rmatrix, ⎛ ⎞ a2 1 0 0 . . . . . . 0 ⎜ 0 a3 1 0 . . . . . . 0 ⎟ ⎜ ⎟ ⎜: : : : :⎟ ⎜ ⎟. ⎜: : : : :⎟ ⎜ ⎟ ⎝ 0 0 0 0 . . . ar 1 ⎠ 1 0 0 0 ... 0 a1 Observe that its determinant is a1 . . . ar + (−1)r+1 = 0 , since ai > 0 for all i and at least one of them is > 1. Define Γ(t, (z1 , . . . , zr )) = (tm1 z1 , . . . , tmr zr ), where m := (m1 , . . . , mr ) is the unique solution to the linear system B · m = (1, . . . , 1). These are rational numbers because B has integer coefficients. Then one has ψ(r) (Γ(t, (z1 , . . . , zr )) = tψ(r) (z1 , . . . , zr ) as wanted. It remains to prove that the solutions of this flow are transversal to all the spheres centred at 0 and they converge to 0 when the time goes to −∞. This is a consequence of the following claim: mi > 0 for every i = 1, . . . , r. To prove this claim we first compute the mi explicitly. We find: 1 − a1 + a1 ar − a1 ar ar−1 + · · · + (−1)r−1 a1 ar ar−1 . . . a3 , 1 + (−1)r−1 a1 a2 . . . ar 1 − a2 + a2 a1 − a2 a1 ar + · · · + (−1)r−1 a2 a1 ar . . . a4 m2 = , 1 + (−1)r−1 a1 a2 . . . ar 1 − a3 + a3 a2 − a3 a2 a1 + · · · + (−1)r−1 a3 a2 a1 . . . a5 , m3 = 1 + (−1)r−1 a1 a2 . . . ar

m1 =

and so on.

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Assume r is odd, r = 2h + 1. It is clear that the denominator qi of each mi = pi /qi is positive. We claim pi > 0 for each i. This is a consequence of the following lemma: 4.6 Lemma. Let b1 , . . . , b2h be positive integers. Then 1 − b1 + b1 b2 − + · · · + b1 b2 . . . b2h > 0 . Proof. We prove this by induction. If h = 1 we have 1 − b1 + b1 b2 = 1 − b1 (1 − b2 ) . If b2 > 1 , then b1 (1 − b2 ) < 0 , so 1 − b1 + b1 b2 > 1 . If b2 = 1, then 1 − b1 + b1 b2 = 1 > 0 . Now suppose (induction hypothesis) the claim holds when we have 2h > 2 numbers, let us prove it for 2h + 2. Set α = 1 − b1 + b1 b2 − + · · · + b1 b2 . . . b2h+2 . Then, α = 1 − b1 (1 − b2 (β)) , where β = 1 − b3 + b3 b4 − + · · · + b3 . . . b2h+2 . By the induction hypothesis one has β > 0. Hence 1 − b2 (β) ≤ 0, so α ≥ 1.
Now we take a cycle of even length r = 2h as in Theorem 4.2, so ai ≥ 1 and at least two ai , say ai1 and ai2 are strictly bigger than 1, with i1 being odd and i2 even in the cycle. In this case the denominator qi of each mi is negative; we claim pi is also < 0, so that mi > 0. We prove this by induction. If h = 1, one has p1 = 1 − a1 , p2 = 1 − a2 and a1 , a2 are both > 1 by hypothesis, so each pi is negative. Similarly, for r = 4 one has p1 = 1 − a1 (1 − a4 (1 − a3 )), and necessarily a1 > 1 or a3 > 1; in either case one has p1 < 0, and similarly for p2 , p3 , p4 . Now assume (induction hypothesis) the claim holds for 2h > 2, and we want to prove it for r = 2h + 2 numbers. We have: p1 = 1 − a1 (1 − ar (β)), where β = 1 − a2h+1 + a2h+1 a2h − + · · · − a2h+1 . . . a3 . By hypothesis one aodd is > 1 (notice that a2 does not appear in the formula for p1 , so this could be the aeven > 1 that we have by hypothesis). If this aodd > 1 is one of the ai appearing in the expression for β, then by the induction hypothesis one has β < 0. Thus 1 − ar (β) > 1, hence p1 < 0. If all the aodd appearing in β are 1, then β = 0 and a1 > 1 by hypothesis, so p1 < 0 as claimed. The proof for
the other pi is similar.  (z1 , . . . , zn )) =  : R×Cn → Cn , Γ(λ, Proof of Lemma 4.3. We construct an action Γ iλs1 iλsn  (e z1 , . . . , e zn ), si ∈ Q, such that ψθ (Γ(λ, z)) = ψθ+λ (z). This implies iλ

ˆ (zj )) = e aj −bj . To define it on an r-cycle, Lemma 4.3. On a 1-cycle, define Γ(λ, consider the r × r-matrix A, ⎛ ⎞ −1 a1 0 . . . ... 0 ⎜ 0 −1 a2 0 ... 0 ⎟ ⎟ ⎜ ⎜ : : : : : ⎟ ⎟ ⎜ ⎜ : : : : : ⎟ ⎟ ⎜ ⎝0 0 0 0 −1 ar−1 ⎠ ar 0 0 ... 0 −1 whose determinant is (−1)r (1 − a1 . . . ar ) = 0. Let s = (s1 , . . . , sr ) be the unique  as above, with these solution to the linear system A · s = (1, . . . , 1) and define Γ

VII.5. Twisted Pham-Brieskorn singularities

191

weights si , which are in Q because A is integral. Doing this for each cycle of σ we obtain a flow with the properties of Lemma 4.3.
4.7 Example. Consider the Pham-Brieskorn polynomial, an , ai > 2 for all i . f (z1 , . . . , z2n ) = z1a1 + · · · + z2n

One has the Milnor fibration of f as given in [168]: f : S4n−1 − M → S1 , ||f || where M is the link. Let F be one of the Hamiltonian vector fields obtained ∂f , . . . , ∂z∂f2n ) by permuting by pairs the partial from the gradient vector field ( ∂z 1 derivatives of f and changing the sign to one derivative in each pair. For example: a2n −1 2n−1 , −a2n−1 z2n−1 F (z1 , . . . , z2n ) = (a2 z2a2 −1 , −a1 z1a1 −1 , . . . , a2n z2n a

−1

).

Then F is holomorphic and its solutions are tangent to the fibres of f . Every such vector field F satisfies the hypotheses of Theorem 4.2, taking X(z) = (z1 , . . . , zn ), so it has an associated fibration “` a la Milnor” given by Theorem 4.2. It would be interesting to understand how these fibrations are related to Milnor’s original fibration.

VII.5

On the topology of the twisted Pham-Brieskorn singularities

We recall that a real singularity is a twisted Pham-Brieskorn singularity if it is defined in Cn by a polynomial map of the form ψ(z) = λ1 z1a1 z σ1 + · · · + λn znan z σn , where each ai ≥ 2, i = 1, . . . , n, the λi are non-zero complex numbers and σ = {σ1 , . . . , σn } is a permutation of the set {1, . . . , n}. We can re-label the variables (z1 , . . . , zn ) so that this singularity takes the form:  ψ(z) = λ1 zσa11 z 1 + λ2 zσa22 z 2 + · · · + λn zσann z n . This can be regarded as being 0 ∈ Cn union the polar variety in Cn − 0 of the nonsingular foliations F and S, where F is the holomorphic 1-dimensional foliation defined by the vector field F (z) = (λ1 zσa11 , . . . , λn zσann ) , and S is the foliation given by the spheres around 0.

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Chapter VII. Real Singularities with a Milnor Fibration

Given this vector field F , define as before φF (z) =

F (z), z : S2n−1 − M −→ S1 , |F (z), z|

where M = V ∩ S2n−1 is the link, which can be regarded as the intersection of V with the unit sphere. The theorem below is essentially a summary of results proved in Chapter VI (Sections 3, 4) and in the previous sections of this chapter. 5.1 Theorem. (i) The link M is a smooth, codimension 2, oriented submanifold of the sphere. (ii) The map φ is the projection of a C∞ (locally trivial) fibre bundle, which defines an open book decomposition of the sphere. (iii) Each fibre is a parallelizable, open manifold of dimension 2n − 2, that can be compactified by attaching its boundary M . (iv) Each pair of antipodal fibres {Eθ , Eθ+π } is naturally glued together along M forming the real analytic variety of points where the vector field eiθ F (z) is tangent to the unit sphere (as a real vector field). (v) In particular, the double of the fibre Eθ (compactified by attaching its boundary) is the link of the real hypersurface given by the real part of the function F (z), z. The obvious next thing we want is to study the topology of the link of these singularities, as well as that of the corresponding fibrations. Alas this is not easy and the only known cases so far are: a) when the permutation σ is the identity; and b) when n = 2. Let us consider first case (a). Following [214] we actually consider, more generally, F and X vector fields of the form F = (z1a1 , . . . , znan ), X = (z1b1 , . . . , znbn ) , such that the corresponding exponents satisfy ai > bi ≥ 1 for each i = 1, . . . , n. Let ψ = ψF,X be defined as before, n  ψ = F (z), X(z) = ziai z¯ibi . i=1

By Theorem 4.2 we know that ψ satisfies the strong Milnor condition at 0, so one has an associated Milnor fibration. Let us set ci = ai − bi for each i. One has: 5.2 Theorem. The singular variety V = ψ −1 (0) is homeomorphic to the Brieskorn variety Vc1 ,...,cn , and the corresponding Milnor fibrations are topologically equivalent. More precisely, there exists a homeomorphism h : (Cn , 0) → (Cn , 0) such that ψ = f ◦ h, where f is the Pham-Brieskorn polynomial: f (z1 , . . . , zn ) = z1c1 + · · · + zncn . The proof is straightforward. Let E ⊂ Cn be the divisor { z1 z2 . . . zn = 0 }, and define h : (Cn − E) → (Cn − E) by: h(z1 , . . . , zn ) = (w1 , . . . , wn ) , where

VII.5. Twisted Pham-Brieskorn singularities

193

2bi

wi = ||zi || ci zi . Then h is clearly a real analytic diffeomorphism that extends to a homeomorphism of Cn into itself. Furthermore, for each i = 1, . . . , n one has: 2bi

ziai z¯ibi = zici ||zi ||

2bi

= (||zi || ci zi )ci = wici .


Hence the theorem.

So these singularities are real analytic singularities which are topologically equivalent to the usual complex analytic Pham-Brieskorn singularities, but they are not analytically equivalent because the corresponding maps have different algebraic multiplicity. 5.3 Remarks. (i) As a special case of Theorem 5.2 we have that if some ci is 1, then V is actually homeomorphic to Cn−1 , because in this case the gradient of the polynomial f is never vanishing, so that the corresponding variety V is non-singular and it is a cone over the standard sphere. Furthermore, V is also embedded in Cn as the cone over the standard (2n − 3)-sphere, because the Milnor fibre is a (2n − 2)-disc in this case. (ii) This theorem also provides a new way for thinking of the Brieskorn manifolds and their Milnor fibres. For instance, Poincar´e’s homology sphere Σ is known to be the Brieskorn manifold M(2,3,5) , i.e., the link of the singularity in C3 defined by {z12 + z23 + z35 = 0 }. By the previous theorem this manifold is homeomorphic (and hence, in this case, also diffeomorphic) to the set of points in C3 where the complex line field spanned by the vector field F = (z13 , z24 , z36 ) is tangent to the unit sphere S5 ⊂ C3 . The corresponding Milnor fibre is the famous (open) manifold E8 and by the previous results, its double is the set of points where the real line field spanned by F is tangent to S5 . Let us consider now the case when n = 2 and the permutation σ is not the identity. The singularities in question are, up to isomorphism of the form: V = {z1p · z¯2 − z2q · z¯1 = 0} . The simplest case is when p = q = k > 1. This was considered in [226] and one has: 5.4 Theorem. The link L of the singularity V = {z2k · z¯1 − z1k · z¯2 = 0} , consists of k + 3 circles in the unit sphere S3 ⊂ C2 , which are fibres of the Hopf fibration S3 → S2 . The monodromy h of the corresponding fibre bundle is the 2πi 2πi periodic map h(z1 , z2 ) = (e− k−1 z1 , e− k−1 z2 ) , of period k − 1, and the genus of the fibres Eθ is: (k − 2)(k + 1) g(Eθ ) = . 2

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Chapter VII. Real Singularities with a Milnor Fibration

  Thus, the link of the hypersurface Re z2k · z¯1 − z1k · z¯2 = 0 is a closed oriented surface in S3 of genus k 2 , equal to the Poincar´e-Hopf index of the vector field F = (z2k , −z1k ). The proof is straightforward and is given in detail in [226], so we do not include it here. The idea is to prove first that the link L is a union of Hopf fibres, which is done by giving an explicit parametrization of it. That the monodromy is as stated is immediate from the construction of the flow in Lemma 4.3. The interesting part is the computation of the genus of the fibres. Since L is a union of Hopf-fibres, the complement of the link L in S3 is a product (S2 − {points}) × S1 , so it also fibres over S2 . Thus one has that S3 − L is being fibred over S2 − {points} in two different ways: one is via the Hopf fibration, the other via the fibration in Theorem 5.4. In both cases the fibres are transversal to the orbits of the Hopf flow. Using this we get a ramified covering projection from the fibres in Theorem 5.4 to the fibres of the Hopf fibration regarded as a map S3 − L → S2 − {points}. Then Hurwitz’s formula gives the genus of the fibres in Theorem 5.4. This result is anyhow a special case of the results in [202] that we describe in the following chapter, where we study the singularities V = {z1p · z¯2 − z2q · z¯1 = 0} .

VII.6

Stability of the Milnor conditions under perturbations

A natural question to ask is whether the Milnor conditions are stable under perturbations by high order terms. More precisely, if a given real singularity, say defined by f : Rm → R2 , satisfies the (strong) Milnor condition and f ∗ is obtained from f by adding to it terms of sufficiently high order, does f ∗ satisfy the (strong) Milnor condition, and are these singularities topologically equivalent? This was considered and answered affirmatively in [214], where precise orders for the perturbations allowed are also given for the quasi-homogeneous singularities, as for example the twisted Pham-Brieskorn real singularities. The key for this is to re-interpret these conditions in terms of finite-determinacy of map-germs, using [261, 117, 212, 213]. Here we only make a few comments about this and state some of the results in [214]. We refer to the literature for details. Let C(n, p) be the space of smooth map-germs from (Rn , 0) into (Rp , 0), and let J k (n, p) be the set of k-jets {j k } of elements of C(n, p). Let R be the group of germs of C ∞ -diffeomorphisms (Rn , 0) → (Rn , 0); R acts on C(n, p) by composition on the right. An element f ∈ C(n, p) is k-R-determined if the R-orbit of f contains all germs whose k-jet at 0 coincides with the k-jet of f . Similarly, one has the group C l -R, of local diffeomorphisms of class C l , l > 0, or homeomorphisms if l = 0, which acts on the corresponding space C l (n, p). We will consider rather the induced equivalence relation on C(n, p) and the corresponding notion of

VII.6. Stability of the Milnor conditions

195

k-C l -R-determinacy. One says that f is C l -R-finitely determined if it is k-C l -Rdetermined for some k. Let IR (f ) be the ideal of C(n) generated by the p × p-minors of the Jacobian matrix of f and NR (f ) = det{(dfx )(dfx )t } = sum of squares of p × p-minors of dfx . We say that NR (f ) satisfies a Lojasiewicz condition of order r (> 0) if there exists a constant c > 0 such that NR (f ) ≥ c|x|r . The condition that f be C l -R-finitely determined for 0 ≤ l < ∞ is equivalent to the condition that NR (f ) satisfies a Lojasiewicz condition for some r. Moreover, this inequality provides precise estimates for the degree of C l -R-determinacy of f . This result was discovered by Kuo in [116] for the case p=1. The extension to higher dimensions was considered by many authors. (See [261] for a complete account of the subject, and [212] for precise estimates on the degree of C l -determinacy of f .) We recall that f ∈ C(n, p) analytic satisfies the Milnor condition at 0 if its Jacobian matrix Df has rank p everywhere on a punctured neighborhood of 0 ∈ Rn . The theorem below is implicit in [261] (see also [212], Proposition 2.4.d): 6.1 Theorem. Let f ∈ C(n, p) be analytic. Then f satisfies the Milnor condition at 0 if and only if f is C l -R-finitely determined for every l ∈ [0, ∞). Proof. For analytic germs, NR (f ) satisfies a Lojasiewicz condition at zero if and only if the variety of the ideal IR (f ) reduces to 0 ([261], Lemma 6.2), and this is clearly equivalent to the Milnor condition for f . Now the result follows, for instance, from [212], Proposition 2.4.
As a consequence one has: 6.2 Corollary. Let f ∈ C(n, p) be analytic and assume it satisfies the Milnor condition at 0. Then every perturbation of f by terms of sufficiently high order also satisfies the Milnor condition at 0 and is topologically equivalent to f . When we consider a real analytic function from Cn into C, then regarded as a function from R2n into R2 , its real and imaginary parts are f1 = Re f = 12 (f + f ) 1 (f − f ) , respectively. So f1 is weighted homogeneous if and and f2 = Im f = 2i only if f2 is weighted homogeneous, and in this case they have the same weights and total degree. As mentioned above, for singularities as in Corollary 6.2 which are quasi-homogeneous, one can get rather precise estimates of the least order we need in the perturbations to guarantee that the Milnor condition is preserved (see [214]). Regarding the strong Milnor condition, it was proved in [214] that if f ∈ C(2n, 2) is analytic and its component functions f1 and f2 satisfy conditions A and BR at 0 (hence f satisfies the strong Milnor condition), then there exists a positive integer N such that for every map-germ f ∗ whose N th jet at 0 coincides with that of f , j N f ∗ (0) = j N f (0), the component functions f1∗ and f2∗ of the map-germ f also satisfy conditions A and BR at 0.

196

VII.7

Chapter VII. Real Singularities with a Milnor Fibration

Remarks and open problems

We know from the previous discussion that the twisted Pham-Brieskorn singularities, ψ(z) = λ1 z1a1 z σ1 + · · · + λn znan z σn , are a class of real singularities which behave very much as complex singularities, in the sense that their links define open book decompositions of the spheres. It would be interesting to know more about the topology of these singularities. As mentioned before, so far we only understand the cases when the perturbation σ is the identity (the corresponding singularity is topologically equivalent to a usual Pham-Brieskorn singularity) and the case n = 2 that we shall describe in the following chapter. A consequence of the results in [202] that we present in Chapter VIII is that for n = 2 the links are isotopic to the links of singularities of the form f g¯, where both f, g are holomorphic functions C2 → C with no common branch. However that can only happen in this dimension, because the twisted Pham-Brieskorn singularities have an isolated singularity, while the varieties {f = 0} and {g = 0}, with f, g holomorphic, must intersect in a variety of dimension more than 0, if n > 2. There are several open problems in this respect which I believe are interesting. Essentially everything we know that holds for complex singularities is likely to have a counterpart for these twisted singularities. For instance, one can stabilize them adding terms zj2 in new variables, and presumably the new link is the suspension over the previous one, one may look at the signature of the quadratic form of its Milnor fibre, etc. Here are some of the problems that I consider fundamental in this theory. Problem 1: Study the topology of the link a) For general reasons, for n > 2 the link M 2n−3 is necessarily connected; how connected is it? For instance, in the complex analytic case it is (n − 3)connected. What about its homology? In particular, it would be interesting to know when the link is a homology (homotopy) sphere. b) Is one getting new manifolds in this way, which are not links of complex singularities? (either up homeomorphism or diffeomorphism) What about the algebraic knots (S2n−1 , M ), where M is the link? For instance, when n = 1 the link M is necessarily a finite union of circles embedded in the 3-sphere, so the topology of M by itself is not exciting; what is interesting is the corresponding algebraic knot (S3 , M ). As we prove in the following chapter, the pairs we get in this way for the twisted Pham-Brieskorn singularities are isotopic to links coming from complex singularities, but the various components get different orientations and this causes the corresponding Milnor fibrations to be different. In dimension n = 2 the link is a Seifert 3-manifold and one should be able to compute its Seifert invariants from the weights and the twisting σ (c.f. Chapter III). Then, using [186] one can decide whether or not these are links of complex

VII.7. Remarks and open problems

197

singularities. But again, “almost” every Seifert manifold is homeomorphic (maybe reversing orientation) to a complex singularity link (see for instance [184]); however the Milnor fibrations one gets are sure not to be (in general) equivalent to the Milnor fibrations of complex singularities. On the other hand we know from the previous chapter that the 3-torus does arise from a real singularity of the type we are considering (though it does not have a Milnor fibration) and it is not a complex singularity link. And what about higher dimensions? Of course a way for studying the topology of the link is to follow Milnor’s program in [168], and study the link by looking at the Milnor fibre and the corresponding monodromy. This brings us to: Problem 2: Study the topology of the fibres The only cases I know so far are when the twisting σ is the identity, and the singularity is topologically equivalent to a complex one, so its fibre has the homotopy type of a bouquet of spheres of middle dimension, or when n = 2 and for general topological reasons the fibre must have the homotopy type of a bouquet of 1-spheres, i.e., circles. Are these special cases of a more general theorem?; has the fibre in general the homotopy of a cw-complex of middle dimension? A hint may be given by a surprising result in VIII.3.2 below, which generalizes Theorem 5.4 above, stating that for n = 1 the double of the Milnor fibre of the singularity z1p z¯2 + z2q z¯1 = 0 is p · q, the index of the vector field (z1p , −z2q ) that defines it. On the other hand I believe it is sensible to think that in all these cases the fibres are homeomorphic to Stein manifolds and therefore have the homotopy type of CW-complexes of middle dimension (c.f. the next problem). Problem 3: Understand the relation with the singularities f g It is obvious that every twisted Pham-Brieskorn singularity is defined by a linear combination of functions of the type f g with f, g holomorphic, and we know from Theorem I.6.4 that whenever f g has an isolated critical value, one has a Milnor fibration. Notice however that since f and g are holomorphic, their zero-sets are hypersurfaces in Cn and therefore they must intersect in an analytic space of complex codimension at most 2. This means that for n > 2 the hypersurface Vf g = f g−1 (0) necessarily has non-isolated singularities. This emphasizes the remarkable regularity one has for the twisted Pham-Brieskorn singularities, where you always get isolated singularities. This is surely a special case of a more general theorem about linear combinations of functions of the type f g. Notice that for n = 2 the only possible cases are (up to scalars) z1p z 1 + z2q z 2 and z1p z 2 + z2q z 1 . We already know that in the first case these singularities are topologically equivalent to the Pham-Brieskorn singularities z1p−1 + z2q−1 . In the next chapter we will see that the second type of singularities have links isotopic to the links defined by z 1 z 2 (z1p+1 + z2q+1 ). Thus both cases yield links topologically equivalent to links of singularities f g; but as remarked above, this cannot happen in higher dimensions.

198

Chapter VII. Real Singularities with a Milnor Fibration

The following problem is not fundamental for the study of these singularities, but if there is a positive answer to it, this would open a very interesting line of research, inspired in [151, 161, 162, 160]. I do not expect that answering Problem 4 should be too difficult, but I have not been able to do it. Problem 4: Study the relation with complex geometry More precisely, if M is the link of a twisted Pham-Brieskorn singularity and N = M/S1 is the quotient of M by its canonical S1 -action, is N (canonically) a complex manifold? (maybe with some conditions on the weights and the twisting). We recall from Chapter VI that if we consider the singularities V = { λ1 z1 z¯1 + λ2 z2 z¯2 + · · · + λn zn z¯n = 0 } with the λi satisfying certain conditions (that their convex hull in C contains the origin and they satisfy a weak hyperbolicity condition) then V ∗ = V − 0 is a real analytic manifold in Cn of codimension 2 and transversal to the holomorphic foliation F defined by the vector field F = (λ1 z1 , . . . , λn zn ). Hence the transversal holomorphic structure of F endows V ∗ with a complex structure. The canonical C∗ -action on V ∗ is by holomorphic transformations and the quotient N is a complex manifold, diffeomorphic to the quotient M/S1 , where M is the link and the S1 -action is the restriction of the C∗ -action. These are the LVM-manifolds studied in VI.2. For the twisted Pham-Brieskorn singularities one has a similar picture and the question is to decide whether (maybe with some extra conditions) the variety V ∗ is transversal to the corresponding holomorphic foliation.

Chapter VIII

Real Singularities and Open Book Decompositions of the 3-sphere In the previous chapter we introduced the singularities of the form ψ(z) = λ1 z1a1 z σ1 + · · · + λn znan z σn and proved that if all ai are > 1, then these singularities behave in some respects as complex singularities, in the sense that they define open book decompositions of the sphere, and the corresponding projection map (Sε2n−1 − M ) −→ S1 , where M is the link, is given by ψ/|ψ|. We proved that if the permutation σ is the identity, then these singularities are topologically equivalent to usual Pham-Brieskorn singularities. This chapter is based on [202]. Here we study the other possible case when n = 2, i.e., singularities of the form, z1p z 2 + z2q z 1 = 0 ,

p, q > 1 .

We find the resolution graph of these singularities and from it, using [184, 200], we compute the genus of the pages of the corresponding Milnor fibration and its monodromy, thus giving the complete topological description of these fibrations. For this we find a “topological” resolution π : X → C2 of the singularity {z1p z 2 + z2q z 1 = 0 } via the usual technique for studying complex plane curves, i.e., by performing appropriate blow-ups (Sections 4 and 5). The additional problem we face is that, since the singularities in question are only real analytic, we have to employ a “trick” to transform them by a homeomorphism, in some step of the resolution process, in order to get a “divisor” with normal crossings. In this way we obtain a topological resolution of f and then a decorated plumbing graph, which gives the isotopy class of its link and the topology of these fibrations (using [184, 200]).

200

Chapter VIII. Open Book Decompositions of the 3-sphere

Sections 1–3 are a brief exposition of results in [75, 184, 259, 200] about resolution of complex planes curves, the resolution graphs and the classification of horizontal open book fibrations, restricting the discussion to Seifert links, which is all we need (instead of the more general Waldhausen links). The results of [202] that we present here prove that the link of the singularity z1p z 2 + z2q z 1 = 0 is orientation-preserving isotopic to the link of z 1 z 2 (z1p+1 + z2q+1 ) = 0 , which is a singularity of the form f g¯, with f, g holomorphic functions. Then [200] implies that the corresponding open book decompositions are topologically equivalent. In Section 7 we speak briefly about singularities of the type f g¯ in general, and about the results of [199] in this respect.

VIII.1

On the resolution of embedded complex plane curves

We recall that in IV.5 we spoke about resolution of singularities, and particularly of curves and surfaces. Here we review more carefully the case of resolution of plane curves, i.e., complex curves in C2 . This material is classic and there are several excellent expositions of the topic, as for instance [249], [44, III.8.4,5] and Chapter V in [75], so we only sketch the arguments, both for completeness and to introduce notation and arguments that we use later in this chapter. Let X = C2 and consider its germ at 0. The set of lines through 0 is CP1 .  given by The blowup of X at 0 is the space X  = {(z, ) ∈ C2 × CP1 | z ∈  } , X  → X. If z = 0, then there is exactly one  = z ∈ C2 with projection π : X passing through z and 0, so π −1 (z) consists of the single point (z, z ), while one has π −1 (0) = E ∼ = CP1 . Notice that if we let z = (z1 , z2 ) denote the coordinates  is the in C2 and [w1 : w2 ] the homogeneous coordinates of points in CP1 , then X 1 2 subset of (z, ) ∈ C × CP defined by the equation: z1 w2 = z2 w1 ,  is a smooth which has no critical points since w1 , w2 cannot be both zero. Hence X 1 2 submanifold of C × CP .  with two coordinate charts, corresponding to the It is customary to cover X 1 usual charts for CP . These are:     | w1 = 0 ; with coordinates v1 = z1 ; u1 = w2 w1 }; U1 = { (z1 , z2 ), [w1 , w2 ] ∈ X     | w2 = 0 ; with coordinates v2 = z2 ; u2 = w1 w2 }. U2 = { (z1 , z2 ), [w1 , w2 ] ∈ X

VIII.1. Resolution of complex plane curves

201

 is obtained Both charts U1 and U2 are biholomorphic to C2 . This means that X 2 by taking two copies of C , U1 and U2 , and gluing them away from the axis by the transition (or attaching) function for these two charts: u2 = u−1 ; and v2 = u1 v1 . 1 Notice that the equation u2 = u−1 1 is the transition function for the usual decom1 in two coordinate charts. CP position of S2 ∼ = This process of blowing up is of course local, so that we may now pick a  and blow it up to obtain a new complex preferred point (z1 , 1 ) ∈ CP1 ⊂ X   surface X2 with a projection π2 : X2 → X1 = X, and so on. We now want to use this process of blowing ups to describe the embedded resolution of singularities of curves in C2 . By this we mean a non-singular complex s obtained by a finite number of blow-ups as above, together with the surface X s → X1 = X, such that the proper (or strict) transform projection map πs : X   C ⊂ Xs of the germ at 0 of a curve C ⊂ C2 is non-singular; in a neighborhood of the exceptional divisor E = πs−1 (0) it is a disjoint union of r discs, where r is the number of branches of C at 0. Moreover, we want E to be a “normal crossings  meets E transversally divisor”, as explained in IV.5, and that each component of C in a smooth point of E. 1.1 Example. Consider the map f : C2 → C defined by (z1 , z2 ) → z1 · z2 and let C = f −1 (0), so that C consists of the two coordinate axis {z1 = 0} and {z2 = 0}. The intersection C ∩ S3 is the Hopf link in S3 . This is the standard double point, which is resolved by a single blow-up, since this transformation separates the lines through the origin. To see this analytically, let U1 , U2 be the above coordinate  the blow-up of C2 at 0, and let π be the projection map. In the chart charts of X, U1 corresponding to u1 = 0, the projection π is given by π(v1 , u1 ) = (v1 , u1 ·v1 ), so that π −1 (C)∩U1 is given by the equations u21 v1 = 0, and u1 = 0 by hypothesis, so one has v1 = 0. Similarly, on U2 one has π(v2 , u2 ) = (u2 · v2 , v2 ) and π −1 (C) ∩ U2 is given by u2 v22 = 0 with u2 = 0, so v2 = 0. Thus the proper transform of C  which is a copy of C, union v2 = 0 in consists of the curve v1 = 0 in U1 ⊂ X,   are disjoint U2 ⊂ X, which is another copy of C. Both of these copies of C in X −1 and they meet the divisor E = π (0) transversally at each point. It is clear that if we had, instead of the double crossing at 0 given by the two coordinate axis of C2 , r lines passing in C2 through the origin, then we could resolve this singularity with a single blow-up; we would get r disjoint copies of C  and meeting the divisor E transversally. embedded in X 1.2 Example. Consider now the singularity in C2 given by: { (z1 , z2 ) | z12 + z23 = 0 } , whose link is the trefoil knot. It is an exercise to show that in this case the singularity is again resolved with a single blow-up. However this resolution is not entirely 1 is tangent to the exceptional curve satisfactory because the proper transform C

202

Chapter VIII. Open Book Decompositions of the 3-sphere

E1 . By performing a second blow-up one gets a resolution where the exceptional divisor E2 consists of two copies of CP1 meeting transversally in a double point 2 , which is of course smooth, meets E precisely at P , and the proper transform C P ; i.e., the total transform consists of three non-singular curves meeting at one point. With a further blow-up we remove this triple crossing. We get a normal crossings divisor E3 with three components, and the proper transform intersects E3 transversally at a smooth point of E3 . 1.3 Example. Consider, more generally, the singularity in C2 given by: { (z1 , z2 ) | z1p + z2q = 0 } ; 2 ≤ p ≤ q .  as before. Notice that if p = q, then C consists of p We consider the blow-up X lines through the origin and therefore a blow-up resolves them as in the previous Example 1.1. So we may assume p < q. The equation for π −1 (C) ∩ U1 is: v1p + (u1 v1 )q = v1p (1 + uq1 v1q−p ) = 0 ; and the equation for π −1 (C) ∩ U2 is: (u2 v2 )p + v2q = v2p (up2 + v2q−p ) = 0 . (Notice that this also applies for the case p = q and confirms that the resolution consists of p disjoint complex lines that meet the divisor transversally.) We set  C11 = C 1 ∩ U1 and C21 = C 1 ∩ U2 . Since p < q, C11 does not meet the line C 1 = C, {v1 = 0} and therefore it is non-singular. Let us see what happens at the second coordinate chart U2 . There are three cases to consider according as q − p is <, =, or > p. When q − p = p, the curve C21 consists of p lines which meet at 0 ∈ U2 and this can be resolved by a blow-up as in Example 1.1. In the other two cases the curve C21 is in general singular, or (as in the above case p = 2, q = 3, it is smooth but tangent to the divisor), but in all cases one can prove that the singularities have been “simplified” in a certain way that we will not make precise here, and we refer to the literature for this. The point is that one arrives at the following theorem, already stated in IV.5: 1.4 Theorem. (Embedded good resolution) Let C ⊂ C2 be a germ of a complex analytic curve with a singularity at 0. Then there exists a non-singular complex  and a proper morphism π : X  → C2 which is a composition of a finite surface X number of blow-ups, such that: (i) The exceptional divisor E has non-singular irreducible components, all copies of CP1 , which meet normally; thus each singularity of E, if there is more than one irreducible component, is an ordinary double point.  of C in X  (i.e., the closure of π −1 (C −0)) consists of (ii) The proper transform C a finite number of disjoint complex lines that meet E transversally at smooth points of E.

VIII.1. Resolution of complex plane curves

203

There are several methods for describing the (embedded) resolution of a singularity, one of them is by their resolution graphs. These are similar to the graphs used in Chapter IV to describe surface singularities. Given the germ at 0  → C2 be a good embedded resolution, of a complex plane curve C ⊂ C2 , let π : X obtained by a sequence of blow-ups; let E = π −1 (0) be the exceptional divisor,  be the E = E1 ∪ · · · ∪ Er its decomposition in irreducible components, and let C  = C 1 ∪ · · · ∪ C s its connected components. So each proper transform of C, with C  with a certain self-intersection number −wi = Ei is a copy of CP1 embedded in X Ei2 , determined by the stage in which Ei appeared in the sequence of resolutions that define π; this weight, an integer, measures the Euler number of the normal  Each C i is a complex line in X  that meets E transversally at bundle of Ei in X. a smooth point in one of the components Ei . Given this information, we form the  as follows: to each Ei we associate a vertex vi with a weight, resolution graph of X which is −wi = Ei2 ; two vertices are joined by a line if the corresponding curves intersect. All vertices represent now curves of genus 0. By performing plumbing  and the according to this graph, as in Chapter IV, we recover the topology of X  Blowing down E to a point we obtain C2 way the divisor E is embedded in X.  → which is the resolution map. and a projection π : X s : for each C j we draw an  = C 1 ∪ · · · ∪ C It remains to describe the curve C  arrow based at the vertex vi that represents the curve Ei where Cj meets E. When s project into the s branches of C. Since the 1 , . . . , C we blow down E, the lines C projection π is a biholomorphism away from E, one has that for every sufficiently small sphere S3ε around 0 ∈ C2 , the pair (S3ε , L), where L = C ∩ S3ε is the link of C,  ∂X  ∩ C),  where ∂ X  = π −1 (S3ε ) is the boundary is diffeomorphic to the pair (∂ X, of a regular neighborhood of the exceptional divisor E. Therefore, the topology of the link L in S3ε is entirely determined by the resolution graph. Thus, the resolution graph consists of a plumbing graph, where all genera  and a number of arrows that are 0, which describes the topology of the space X, represent the components of the link. Thus a resolution graph is called in [75, p. 134] a decorated plumbing graph (a tree in this case). The point now is how to construct the resolution graph of a plane curve. In general, this can be done using the Puiseux parameterization of (irreducible) plane curves: it is well known that each germ of an irreducible complex analytic curve C (or more generally, each branch of such curve) is homeomorphic to (C, 0). Furthermore, if C is defined by f (x, y) = 0 then, making a change of coordinates if necessary, one can solve this equation for y in terms of x, obtaining a solution of the form: y = a1 xm1 /n1 + c2 xm2 /n2 + · · · , called a Puiseux series for C, or a Puiseux parameterization C → C, where the ai are non-zero complex numbers, each pair (mi , ni ) are relatively prime positive integers and m1 /n1 < m2 /n2 < · · · . The existence of these parameterizations was first shown by Newton, using what is now called the Newton polygon, but it was Puiseux who investigated these parameterizations with more detail. The (mi , ni )

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Chapter VIII. Open Book Decompositions of the 3-sphere

are the Puiseux pairs of the irreducible curve C. The Puiseux pairs determine the topology of C: we know already from Chapter I that C intersects transversally every small sphere around 0 and C is locally embedded in C2 as the cone over its intersection K = C ∩ S3ε , which is a knot. The Puiseux pairs tell us exactly what type of knot it is: the first Puiseux pair gives a torus knot K1 of type (m1 , n1 ), the second gives a cable knot of the previous one, i.e., a knot K2 embedded in the boundary of a tubular neighborhood of K1 , and the pair (m2 , n2 ) describes this embedding, and so on. One has that all but finitely many terms of the power series can be removed without changing the topology of the knot. Thus we conclude that the knots one gets in this way are all iterated torus knots, described by the Puiseux pairs (see for instance [44, 8.3] or the Appendix to Chapter I in [75] where the construction of these knots out of the Puiseux pairs is beautifully explained.) The Puiseux pairs determine a resolution graph of the singularity (see, for instance [44, 8.4, Th. 15]). In the following sections we give examples of this.

VIII.2

The resolution and Seifert graphs

This section is a brief summary of material in [75, 184], which are our basic references here. We recall (see III.1.10) that a compact oriented 3-dimensional manifold M is a Seifert manifold if it admits an effective S1 -action. The orbits of this action give a fibreing π : M → B = M/S1 , where B is a compact, connected 2-manifold with or without boundary (according as M has boundary or not). We also recall that a link in M means a disjoint, finite union of circles embedded in M . We assume further that the base B is orientable. A link L in M is reducible if M can be expressed as a non-trivial disjoint sum, i.e., M can be expressed as a connected sum M = M1 #M2 and this decomposition separates L. Otherwise we say that L is irreducible. 2.1 Definition. An irreducible link L in a manifold M is a Seifert link if it is a union of Seifert fibres of some Seifert fibration of M (c.f. [75, Chapter II]). In the sequel, we avoid considering Seifert links whose complement in M is a solid torus or a product (torus × [0, 1]). These degenerate cases do not appear among the links of singularities considered in this work, except in the special cases discussed in the remark at the end of of Section 5. 2.2 The Seifert graph. Given a Seifert link L, the uniqueness theorem of Waldhausen ([259] or [109, Theorem VI. 18]) implies that there exists a unique Seifert fibration of M , up to isotopy, for which L is a union of Seifert fibres, and the isotopy class of L is characterized by its Seifert graph G(M, L). This graph G(M, L) has a single vertex, to which one associates a weight and attaches arrows and stalks, each with an associated pair of numbers (α, β). This is done as follows. For each component of L (respectively for each exceptional Seifert fibre which is not a component of L), one attaches to the vertex an arrow (respectively a stalk),

VIII.2. The resolution and Seifert graphs

205

whose extremity is weighted by the pair (α, β) of its corresponding normalized Seifert invariants (0 ≤ β < α). These numbers are defined by the equation αa + βb = 0 in H1 (N, Z), where N is a small tubular neighborhood of the component of the link (or of the corresponding exceptional fibre), saturated with Seifert fibres, b is an oriented Seifert fibre on ∂N and a is an oriented curve on ∂N such that the intersection a · b is +1 on ∂N , oriented as the boundary of N . The vertex of the graph is endowed with a weight, which is the rational Euler number e0 = e(M → B) of the Seifert fibration, introduced in (III.1.10). To define e0 one defines first the usual Euler number e of the Seifert fibration. We recall that if E is an oriented S1 bundle over an oriented 2-dimensional, compact, connected, manifold B, then its usual Euler class is the primary (i.e., non-automatically zero) obstruction for constructing a section of E (c.f. IV.1). This class lives in H 2 (B; Z) and it becomes a number when we evaluate it on the orientation class of B. If B has non-empty boundary, then H 2 (B; Z) ∼ = 0, so the bundle is trivial. However, if we fix a choice of a trivialization of E over ∂B, i.e., a section of τ : ∂B → E, then one has an Euler class of E relative to τ , e(E; τ ) ∈ H 2 (B, ∂B; Z) ∼ = Z; evaluating e(E; τ ) on the orientation cycle of the pair (B, ∂B) we obtain an integer, which is by definition, the Euler number of E relative to τ . Now, given an oriented Seifert fibration π : M → B on a 3-manifold M , let us remove from B small, pairwise disjoint, open discs around the points corresponding to the special fibres, and denote by B0 what is left. Let E be π −1 (B0 ), which is M minus a union of open solid tori. This is an S1 bundle over B0 . On each boundary torus Ti , one can choose a unique (up to isotopy) oriented curve a which intersects each Seifert fibre in exactly one point and satisfies that m = α[a] + β[b], where m is a meridian of Ti , (α, β) are the corresponding reduced Seifert invariants, and [b] is the homology class represented by one Seifert fibre. This curve a determines a section of E|Ti . Doing this for each boundary torus we obtain a section of E over ∂B0 . The Euler number e = e(M ) of the Seifert fibration π : M → B is defined to be the Euler number of E relative to the given trivialization over ∂B0 . Then the rational Euler number of the Seifert fibration, which is the weight of the vertex in the Seifert graph, is defined by: e0 = e −

d  βi . α i=1 i

The number e0 has important geometric properties and it has been used by several authors. For instance, in this work it appeared already in III.2.ii, where it was denoted by e(M → B), following the notation of [186]. This number was introduced in [187, 204] for Seifert manifolds in general and in [204, 186, 184] it is shown that a closed Seifert manifold is the link of a surface singularity iff e0 is negative. The Seifert graphs provide a more compact way of describing the resolution graphs of Seifert links; all the data of a resolution graph is encoded in the Seifert

206

Chapter VIII. Open Book Decompositions of the 3-sphere

graph, and the way for passing from one to the other is standard, and very well explained in [184, Th. 5.6]. For example, Figure 12 represents the Seifert graph of the torus knot (2, 3) obtained from the complex singularity z12 + z23 = 0. (1, 0)

(2, 1)

0, −1/6

(3, 2) Figure 12: Seifert graph

−2

−1

−3 Resolution graph.

These constructions generalize naturally to Waldhausen links, of which the Seifert links are a special case; we refer to [259, 184, 200] for this.

VIII.3

Seifert links and horizontal fibrations

Let us present now some concepts and results of [200] that we need in the sequel. 3.1 Definition. Let L be a link in a 3-dimensional compact oriented manifold M . An open book fibration of L is a C ∞ locally trivial fibration Φ : M − L −→ S1 which equips M with an open book decomposition with binding L. (See I.5 for the definition and basic properties of open books.) In other words, for each component K of the link L there exists an open tubular neighborhood N (K) of K in M − (L − K) and a homeomorphism τ : S1 × D2 −→ N (K) such that for all (t, z) ∈ S1 × D2 one has: z . (Φ ◦ τ )(t, z) = |z| In this case the fibres of Φ are called the pages; each page is a 2-dimensional open surface whose closure in M is a compact surface with boundary L. The link L is called the binding of the open book. In the sequel, all the open book fibrations considered have connected pages. 3.2 Definition. If (M, L) is a Seifert link and if one has an open book fibration Φ of L, then such a fibration is said to be horizontal if the fibres of Φ are transverse to the Seifert fibres, which are circles. According to [259], this transversality is automatically realized for all Seifert links up to isotopy, except in the degenerated cases avoided at the beginning of this section.

VIII.3. Seifert links and horizontal fibrations

207

Now, let Φ : M − L −→ S1 be an open book fibration of the link L ∈ M → − and let K be a component of L. Let us choose an orientation K of K. Let D → − be a meridian disk of N (K) oriented so that one has intersection D · K = +1 in M , and let us equip its boundary m with the induced orientation. One denotes → − by ( K ) ∈ {−1, +1} the degree of the restriction of Φ to the oriented meridian → − → − m. Note that if − K denotes K equipped with the opposite orientation, then − → → − (− K ) = −( K ). If L is a Seifert link and if Φ is horizontal, then there exist two natural → − orientations of the link L. The first one, denoted by L flow is obtained as follows: as the Seifert fibres of M − L are transversal to the fibres of Φ, one can orient each Seifert fibre b by a flow which lifts, via Φ, the unit tangent vector field of S1 = {z ∈ S; |z| = 1} compatible with the complex orientation. As the base of the → − Seifert fibration is orientable, this orientation b flow is the same for each Seifert → − fibre b of M − L and extends to an orientation L flow of L as a union of Seifert fibres. In other words, we may think of the Seifert fibres as being the orbits of an S1 -action on M , and therefore the usual orientation on S1 ⊂ C induces an orientation on the Seifert fibres. On the other hand, we may equip the fibres F of Φ with their natural orien→ − tation, defined by requiring F · b flow = +1. Then the second natural orientation → − of L, denoted by L bound , is the orientation of L as boundary of F . → − It follows from the definitions that ( K bound ) = +1 for each component K → − of L, while ( K flow ) can be ±1. In [200], only the open book fibrations such that → − → − → − L bound = L flow , i.e., ( K flow ) = +1, are studied. 3.3 Definition. Two horizontal fibrations Φ : M −L −→ S1 and Φ : M  −L −→ S1 are topologically equivalent if there exist orientation preserving homeomorphisms H : (M, L) −→ (M  , L ) and ρ : S1 −→ S1 such that: (i) ρ ◦ Φ = Φ ◦ H|(S3 −L) ;

−−−→ → − (ii) For each component K of L, ( K flow ) = (H(K)flow ). → − → − Notice that these conditions imply that H( L flow ) = L flow , i.e., that for each component K of L, the orientation that the flow obtained via Φ induces on K corresponds to the orientation on H(K) given by the flow obtained via Φ . We remark that Lemma 4.5 of [200] extends easily to the situation we consider here and provides a classification of horizontal fibrations of Seifert links where → − the ( K flow ) are not necessarily +1. In fact, [200] deals with Waldhausen links in general, while we consider only Seifert links. Thus condition (i) can be replaced by: (i ) The fibres of Φ and Φ are diffeomorphic and their monodromies are conjugated in the mapping-class group of the fibre. 3.4. The Nielsen Graph. Given a Seifert link (M, L) and a horizontal fibration Φ : M − L → S1 with connected fibre F := Φ−1 (t), considered as an oriented

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surface in M with boundary L, the monodromy of Φ is a diffeomorphism h : F → F defined by the first return map on F of the Seifert fibres, oriented by the flow. This is a periodic diffeomorphism and one can use the work of Nielsen [191] to study it. The Nielsen graph G(h) of h is a complete invariant defined in [200] which classifies the monodromy up to conjugation. We recall that open book decompositions are completely characterized, up to equivalence, by the topology of the fibres and the monodromy maps (see Chapter I). So the Nielsen graph G(h) describes fully the corresponding open book decomposition. Let us recall briefly the construction of the graph G(h). Given an oriented surface S and an orientation-preserving periodic diffeomorphism τ : S → S with order N , denote by O the space of orbits of τ . The projection p : F → O is an nsheeted branched cyclic cover, ramified over a finite number of points P1 , . . . , Pd
, called the exceptional orbits. Let D1 , . . . , Dd
be disjoint open discs in O such that Pi ∈ Di for all i = 1, . . . , d ; let us set Ci = ∂Di for all i = 1, . . . , d ,
ˇ = O − *d Di . Denote also by Ci , i = d + 1, . . . , d + d, the boundary and O i=1 components of O. To each exceptional orbit Pi , i = 1, . . . , d , and to each boundary component Ci , i = d +1, . . . , d +d, of O one associates a triple (mi , λi , σi ) defined ˇ with the orientations induced, via p, from that as follows. Let us endow O and O on F , and we equip each Ci , i = 1, . . . , d +d, with the orientation opposite to that ˇ The integer mi is the number of connected components of of the boundary of O. p−1 (Ci ); then define λi by λi mi = N , and σi is the integer modulo λi defined by ˇ Z) −→ Z/N Z is the homomorphism associated ρ([Ci ]) = mi σi , where ρ : H1 (O, to the N -sheeted cyclic cover p|p−1 (O) ˇ . Then the Nielsen graph G(τ ) consists of a single vertex, weighted by N and by the genus of the quotient surface O, to which d stalks and d boundary-stalks are attached, representing respectively the exceptional orbits and the boundary components of O. The extremity of each stalk or boundary-stalk is equipped with the corresponding triple (mi , λi , σi ). Figure 13 below tells us how to pass from the resolution (or Seifert) graph G(S3 , L) to the Nielsen graph of the corresponding monodromy. This is implied by the following result of [202] which is an extension of ([200], Lemme 4.4), and its proof is exactly the same as that in [200]. The theorem asserts that the Nielsen graph is determined by the Seifert invariants of (M, L) and by the orientation of the components of L regarded as boundaries of the fibres of Φ. 3.5 Theorem. Let Φ : M −L → S1 be a horizontal fibration of a Seifert link (M, L), with connected fibre F , and let h : F → F be the periodic representative of the monodromy of Φ obtained by the first return on F of the Seifert fibres. Assume the resolution graph is as in Figure 13 above. Then: (i) The order N of the monodromy h is: N = − e10

d
+d

i i=d
+1 αi

.

(ii) The points in the orbits space O = F/h that correspond to the exceptional orbits, are the intersection points of the fibre F with the exceptional fibres of

VIII.4. An example

d stalks i = 1, . . . , d

{

209

d stalks i = 1, . . . , d

(αi , βi )

{

(N/αi , αi , βi )

(g, e0 ) d arrows i = 1, . . . , d

{

i (αi , βi )

g, N d boundary-stalks i = 1, . . . , d

Figure 13: Seifert graph G(S3 , L)

{

(1, N, σi )

Nielsen graph G(h).

the Seifert fibration; and the triple (mi , λi , σi ) attached to each exceptional orbit of is given by the corresponding triple (N/αi , αi , βi ). (iii) The boundary component of O corresponding to the component Ki of L is equipped with the triple (1, N, σi ), where σi , modulo N , is given by the equality: − → αi σi − N βi + (Kiflow ) = 0 , where (αi , βi ) are the corresponding reduced Seifert invariants. In particular the projection π : F → F/h provides a description of the fibre F as an n-sheeted cyclic cover over the surface with genus g, branched over d points with branching indices αi , i = 1, . . . , d and N for the d remaining points. Thus we know the topology of the fibre F : it has d boundary components and its genus is obtained from the Hurwitz formula. One gets: 3.6 Corollary. With the notation of Theorem 3.5, the genus of the fibres of Φ is:   d
 N 1  gF = . (N − 1)(d − 2) + N d − 2 α i=1 i So we have that the Nielsen graph G(h) of the horizontal fibration describes fully the open book decomposition, giving us the monodromy map of the fibration and the genus of the fibres. And G(h) is determined by the Seifert invariants of (M, L) together with the orientation of the components of L regarded as boundaries of the fibres of Φ.

VIII.4

An example

As a motivation for the following section, we study here the topology of the singularity f (z1 , z2 ) = z23 · z 1 + z15 · z 2 . Following the method used for studying the topology of complex plane curves (see for instance [75, 134]), let us try to describe the topology of the link L by

210

Chapter VIII. Open Book Decompositions of the 3-sphere

performing a composition π : X −→ C2 of a finite number of blow-ups of points, starting with the blow-up of the origin in C2 . We will see that we need to consider also a homeomorphism θ in such a way that the total transform (f ◦ π)−1 (0) has normal crossings (c.f. IV.5). One then identifies the 3-sphere S3 with the boundary of a small semi-algebraic tubular neighborhood W of the exceptional divisor E = π −1 (0) and L with the intersection of the strict transform of f with the boundary ∂W of W . Let π1 : X1 → C2 be the blow-up of 0C2 and set Ψ1 = f ◦ π1 . In the first coordinate chart of X1 , i.e., over C2 −{z2 = 0}, the blow-up is given by: z1 → z1 z2 and z2 → z2 . The exceptional divisor E1 has equation z2 = 0 and one has: Ψ1 (z1 , z2 ) = (f ◦ π1 )(z1 , z2 ) = f (z1 z2 , z2 ) = z23 z 1 z 2 + z15 z25 z 2 = z23 z 2 (z 1 + z15 z22 ) . In the total transform Ψ1 (z1 , z2 ) = 0, the factor z23 z 2 corresponds to the equation of E1 := {z2 = 0}, while (z 1 + z15 z22 ) = 0 is the equation of a smooth branch S1 of the strict transform of f , namely the complex curve with equation z1 = 0. In the second chart of X1 , i.e., over C2 − {z1 = 0}, the blow-up is given by z1 → z1 and z2 → z1 z2 ; E1 has equation z1 = 0 and (f ◦ π1 )(z1 , z2 ) = z13 z23 z 1 + z15 z 1 z 2 = z13 z 1 (z23 + z12 z 2 ) . We notice that z23 + z12 z 2 = 0 is the equation of a singular real surface S, so we have to resolve this singularity. One branch of S has equation z2 = 0. Unfortunately, the presence of both z2 and z 2 in the equation of S does not allow one to factorize either z2 or z 2 . Therefore it is useless trying to separate the branch z2 = 0 from the other branches of S by performing additional blow-ups. Indeed, let us try an additional blow-up π  : X  → X. In the second chart we have, (Ψ1 ◦ π  )(z1 , z2 ) = Ψ1 (z1 , z1 z2 ) = z15 z 1 (z23 z1 + z 1 z 2 ) , and the factor (z23 z1 + z 1 z 2 ) still involves z2 and z 2 with the same exponents as before, so this singularity cannot be resolved by blow-ups. Thus we start again with the term z23 + z12 z 2 , defining S, and we use a trick: we compose π1 with an orientation-preserving homeomorphism θ : X1 → X1 in order to replace S by a complex plane curve. So we start with the term (z23 + z12 z 2 )  4 and we write it as z 2 ( z2 1 + z12 ); now define the map θ : X1 → X1 by: |z2 | 2

θ(z1 , z2 ) = (z1 ,

z2 1

|z2 | 2

)

in the second chart. This is well defined away from the two complex lines transverse to E1 with equations z2 = 0 and z2 = ∞ and it extends in the obvious way to a homeomorphism from X1 to X1 . This homeomorphism coincides with the identity map on the two lines and it is a diffeomorphism on their complement. In the second

VIII.4. An example

211

chart, the inverse map is θ−1 (z1 , z2 ) = (z1 , z2 |z2 |), and the image of S by θ has equation: z 2 |z2 |(z2 4 + z1 2 ) = 0 , or equivalently z 2 (z2 4 + z1 2 ) = 0 . The term z2 4 + z1 2 defines a complex analytic plane curve, which can be resolved by a finite sequence of blow-ups of points in the usual way. Let π2 : X2 −→ X1 be the blow-up of the point (z1 , z2 ) = (0, 0) of X1 . In the second chart, the exceptional divisor E2 = π2−1 (0, 0) has equation z1 = 0 and the strict transform of θ(S) by π2 has equation: z 2 |z2 | (z12 z24 + 1) = 0 . The factor z 2 corresponds to a smooth branch S2 of the strict transform of f by f ◦ π1 ◦ θ−1 ◦ π2 . The term z12 z24 + 1 = 0 does not intersect the exceptional divisor, so it has no contribution for the topology of L. In the first chart E2 has equation z2 = 0 and the strict transform of θ(S) by π2 has equation z2 2 + z1 2 = 0 , which corresponds to the equation of two transverse smooth complex curves S3 and S4 , which are separated by performing the blow-up π3 : X3 → X2 of their common point. Therefore, if we let π = π3 ◦ π2 ◦ θ ◦ π1 , then the total transform (f ◦ π)−1 (0) has normal crossings and the strict transform π −1 (f −1 (0) − {0}) consists of the four smooth curves Si , i = 1, . . . , 4. The configuration of the divisor (f ◦ π)−1 (0) is represented in Figure 14, each irreducible compact component Ej being weighted by its self intersection in X. Let us now identify π −1 (S3 ) with a small tubular neighborhood W of π −1 (0) in X obtained by a plumbing process. The link L = f −1 (0) ∩ S3 is, up to isotopy, the intersection of S1 ∪ S2 ∪ S3 ∪ S4 with the boundary of W . Therefore L has four components Ki = Si ∩ ∂W, i = 1, . . . , 4, and its isotopy class is encoded in the dual plumbing graph Γ of the divisor (f ◦ π)−1 (0), also represented in Figure 14. As this graph has a single rupture vertex (i.e., a vertex with more than three incident edges or arrows), then the link L is a Seifert link. By using the plumbing calculus of [184], one computes from Γ the Seifert graph G(S3 , L), also represented in Figure 14. − → Before computing the degrees (Kiflow ), let us introduce some definitions and make some remarks. Let C be an irreducible component of the total transform (f ◦ π)−1 (0). As in [134, 1.3.2], define a curvette of C as a smooth complex curve in X intersecting transversally C at a smooth point of (f ◦ π)−1 (0). One defines the multiplicity m(C) of f along C as the degree of the restriction of f to a curvette of C. Remember that in a neighborhood of S1 in X, f ◦ π has the following local expression: (f ◦ π)(z1 , z2 ) = (f ◦ π1 )(z1 , z2 ) = z23 z 2 (z 1 + z15 z22 ) ,

212

Chapter VIII. Open Book Decompositions of the 3-sphere

where z2 = 0 is the equation of E1 and where z 1 + z15 z22 = 0 is that of S1 . Therefore m(E1 ) is the degree of the map z2 → z23 z 2 , that is m(E1 ) = 3 − 1 = +2, and m(S1 ) is the degree of z1 → z 1 , so m(S1 ) = −1. Similarly, m(S2 ) = −1, and m(S3 ) = m(S4 ) = +1 as S3 and S4 appear through holomorphic factors in the local expression of f ◦ π. Now, one remarks that, as in the usual resolution of complex plane curves, the multiplicity of a compact irreducible component of the divisor created by the blow-up of a point P is the sum of the multiplicities of the components of the total transform passing through P . Therefore m(E2 ) = m(E1 ) + m(S2 ) + m(S3 ) + m(S4 ) = 3, and m(E3 ) = m(E1 ) + m(E2 ) + m(S3 ) + m(S4 ) = 7. At this step we can compute already the order of the periodic monodromy h of |ff | . Indeed, by definition, the periodic monodromy is the self-diffeomorphism on a fibre of

f ◦π |f ◦π|

given by the first return map of the flow determined by the Seifert

fibration. Therefore its degree is equal, up to sign, to the degree of |ff ◦π ◦π| restricted to a regular Seifert fibre of (S3 , L) disjoint from L. But one of the main ideas of the plumbing calculus is that a regular Seifert fibre of (S3 , L) is, up to isotopy, the intersection with ∂W of a curvette γ of the rupture component of the exceptional divisor (i.e., that which corresponds to the rupture vertex of the dual resolution graph). Then the order N of h is, up to sign, the degree of the restriction of f to a curvette γ of E3 , i.e., N = m(E3 ) = 7. −→ Let us now compute the degree (K1flow ). Remember again that in a neighborhood of S1 , (f ◦ π)(z1 , z2 ) = z23 z 2 (z 1 + z15 z22 ) . In this local chart, W = {(z1 , z2 ) ; |z2 | ≤ η}, where η

1. Thus,

K1 = S1 ∩ ∂W = {(z1 , z2 ) ; z1 = 0, |z2 | = η}. −→ Let K1C be the knot K1 oriented as the boundary of the complex curve S1 ∩ W . −→ Let us compute first (K1C ). By definition, it is the degree of the restriction of (f ◦ π)/|f ◦ π| to a small meridian of K1 , which is nothing but the degree of the −→ restriction of f ◦ π to a curvette of S1 . Therefore (K1C ) = m(S1 ) = −1. −→ −→ Let us now compare the orientations K1C and K1flow . Let γ be a curvette of E3 and let us orient the Seifert fibre b = ∂W ∩ γ as the boundary of the complex → − → − curve W ∩ γ. As m(E3 ) = 7 is positive, b C = b flow . Moreover, by plumbing −→ calculus, one knows that the orientation of K1C as a Seifert fibre is compatible → − −→ −→ −→ with that of b C . Therefore, K1C = K1flow , and then, (K1flow ) = m(S1 ) = −1. −→ −→ −→ Similarly, (K2flow ) = m(S2 ) = −1; (K3flow ) = m(S3 ) = +1, and (K4flow ) = m(S4 ) = +1. Using Theorem 3.5 one computes the Nielsen graph G(h) of the monodromy − → of |ff | from the Seifert graph G(S3 , L) and from the degrees (Kiflow ), i = 1, . . . , 4. This is represented in Figure 14. In particular, one can recover from Theorem 3.5.i that the order of the monodromy is 7.

VIII.5. The singularities z1p z¯2 + z2q z¯1 = 0

213

(1, 7, 6)

1 (1, 0) −1

E3 −3

−2

(1, 7, 6)

1 −1/6 (1, 0)

(1, 0)

E1

−1

(3, 2)

7

−1

(1, 7, 4)

(1, 7, 5)

Figure 14: The divisor, the Seifert graph and the Nielsen graph.

VIII.5

Resolution and topology of the singularities z1p z¯2 + z2q z¯1 = 0

We now consider the general case of the function f : (C2 , 0) → (C, 0) defined by f (z1 , z2 ) = z1p z 2 + z2q z 1 , with p and q integers, p ≥ q ≥ 2. The arguments of the previous section generalize to the following result: 5.1 Theorem. Given f as above one has: (i) The link L = f −1 (0) ∩ S3 is a Seifert link with k + 2 components, where k = gcd(p+1, q +1). Two of these components are the Hopf link ({z1 z2 = 0})∩S3 , while the others are a torus link of type (p + 1, q + 1). → − (ii) The degree ( K flow ) equals −1 for the two components of the Hopf link, and +1 for each of the k remaining components. f |f |  

(iii) The monodromy of the fibration

: S3 − L −→ S1 has a periodic represen-

tative h whose order is N = kp q − p − q  = q  = q+1 k .

1 k (pq

− 1), where p =

(iv) Each fibre Fθ = ( |ff | )−1 (eiθ ) has genus 12 k(N − 1) = 3

1 2 (pq

p+1 k

and

− 1 − k).

3

(v) The plumbing graph of (S , L), the Seifert graph G(S , L) and the Nielsen graph of h are represented in Figure 15, where p σ1 − N β1 − 1 = 0 and q  σ2 − N β2 − 1 = 0. We remark that we stated the theorem above considering the unit sphere S3 ⊂ C2 for simplicity, but one can replace this by any sphere centred at 0, of arbitrary positive radius, by [227, Proposition 2.1] (see the previous chapter, where we prove that these singularities are quasi-homogeneous). Chapter VII also gives us an explicit representative of the monodromy of this fibration: this is given 2πi(p+1) 2πi(q+1) by the map (z1 , z2 ) → (e− pq−1 z1 , e− pq−1 z2 ). Proof. Let π1 : X1 → C2 be the blow-up of 0C2 . In the first chart, C2 − {z2 = 0}, one has: (f ◦ π1 )(z1 , z2 ) = f (z1 z2 , z2 ) = z2q z 2 (z 1 + z1p z2p−q ) ,

214

Chapter VIII. Open Book Decompositions of the 3-sphere

k components

{

1

(1, N, N − 1)

(1, 0) (1, N, N − 1)

1 (q  , β2 )

(1, 0) −1/p q 

-1

(p , β1 )

N

(1, N, σ2 )

-1

Figure 15: The Seifert graph and the Nielsen graphs.

and z 1 + z1p z2p−q = 0 is the equation of a smooth branch S1 of the strict transform of f , namely the complex curve with equation z1 = 0. Its multiplicity is the degree of the map z1 → z 1 , then m(S1 ) = −1. In the second chart one has, (f ◦ π1 )(z1 , z2 ) = z1q z 1 (z2q + z1p−q z 2 ) , and, just as in Section 4, it is useless to keep making additional blow-ups to resolve the singularity: S = {z2q + z1p−q z 2 = 0 } , due to the presence of both z2 and z 2 in the equation. Thus we use the same trick we did before. Let us write   z2 q+1 z1q z 1 (z2q + z1p−q z 2 ) = z1q z 1 z 2 ( + z1p−q . 2 ) |z2 | q+1 We now compose π1 with the homeomorphism θ : X1 → X1 defined in the second chart by z2 θ(z1 , z2 ) = (z1 , 2 ) |z2 | q+1 out of the two complex lines z2 = 0 and z2 = ∞; this extends as the identity map on 2 these two lines. In the second chart, the inverse map is θ−1 (z1 , z2 ) = (z1 , z2 |z2 | q−1 ), and θ(S) has equation: 2

z 2 |z2 | q−1 (z2 q+1 + z1 p−q ) = 0 , or equivalently z 2 (z2 q+1 + z1 p−q ) = 0 . This is the equation of a complex plane curve, so it can be resolved by a finite sequence π  : X −→ X1 of blow-ups of points in the classical way. As in Section 4, after one blow-up the term z 2 gives rise to

VIII.5. The singularities z1p z¯2 + z2q z¯1 = 0

215

a smooth branch S2 transverse to the new component of the exceptional divisor which has multiplicity m(S2 ) = −1. After a finite sequence of additional blow-ups one obtains k := gcd(p + 1, q + 1) other branches S3 , . . . , Sk+2 , all transverse to the same component C of the exceptional divisor. To identify the link in S3 defined by these k components of L we observe that in the chart {z1 = 0} of the blow-up X1 , the equation z2q+1 + z1p−q = 0 is the equation of the strict transform of the holomorphic curve z2q+1 + z1p+1 = 0 , whose link is well known to be a torus link q+1 −1 3 of type k( p+1 (Sε ) with a k , k ), by [38] (see also [168]). Therefore, identifying π −1 tubular neighborhood W of the divisor π (0), the link L has k + 2 components Ki = Si ∩ ∂W , two of them, K1 and K2 consisting of the Hopf link, and the q+1 remaining components K3 , . . . , Kk+2 form a torus link of type k( p+1 k , k ). This determines the resolution (plumbing) graph of the singularity. Again, using classical computations of Seifert invariants and the plumbing calculus of [184], one obtains from this the Seifert graph G(S3 , L): there are two exceptional Seifert fibres, which are K1 and K2 , with Seifert invariants respectively α1 = (p + 1)/k = p and α2 = (q + 1)/k = q  . As the ambient space is the 3-sphere, then the rational Euler class of the Seifert fibration is e0 = − p
1q
and the two classes β1 mod p and β2 mod q  are related by p β2 + q  β1 = 1 (see [110] for more details). − → As in Section 4, one obtains the weights (Kiflow ), i = 1, . . . , k + 2 by computing the multiplicities m(Si ), i = 1, . . . , k + 2 of the branches of the strict trans−→ −→ − → form of f by π. These are (K1flow ) = (K2flow ) = −1, and (Kiflow ) = +1, i = 3, . . . , k + 2. As in Section 4, we can already compute the order N of the periodic monodromy of f /|f | by computing the multiplicity of the rupture component of the exceptional divisor. In the complex case z1 z2 (z1k+1 + z2k+1 ) this multiplicity equals kp q  + p + q  , the term kp q  coming from the k branches of z1k+1 + z2k+1 , and the terms p and q  from the two branches z1 and z2 . In our real case, the computations of the multiplicities following the sequence of blow-ups are the same as in the complex case, except that the multiplicities of the branches corresponding to the Hopf link (i.e., the branches z1 and z2 ) are counted negatively. Therefore, N = kp q  − p − q  . Using Section 3, one obtains the Nielsen graph and the genus of the fibre. In particular, one can recover the order N of the monodromy from the formula (i) from Theorem 3.5. We notice that in this case, the corresponding Seifert decomposition of the 3-sphere has only two exceptional fibres (the Hopf link), and both of them are components of L. Hence, in the Nielsen graph of the monodromy, all the stalks are boundary-stalks.
In order to state the following result we recall the constructions of the previous chapter and notice that the function f (z1 , z2 ) = z1p z 2 + z2q z 1 in Theorem 5.1 can be regarded as the Hermitian product of the vector fields ξ = (z2q , z1p ) and ι = (z1 , z2 ). 5.2 Corollary. With the hypothesis and notation of Theorem 5.1, each pair of antipodal fibres Fθ and Fθ+π is naturally glued together along the link L forming a smooth real analytic surface Sθ in S3 . The genus of Sθ is p · q, the Poincar´e-Hopf

216

Chapter VIII. Open Book Decompositions of the 3-sphere

index at 0 of the vector field ξ, and Sθ is diffeomorphic to the set of points where the real line field spanned by ξ is tangent to S3 . Proof. The only new statement here is the claim that the genus of Sθ is p · q. This follows from Theorem 5.1 since each fibre Fθ has genus 12 (pq − 1 − k) and it has k + 2 boundary components. When we glue two such fibres along their boundary to get Sθ , we create k + 1 handles. Hence the genus of Sθ is: g(Sθ ) = 2g(Fθ ) + (k + 1) = p · q , as stated. Finally, that this number is the Poincar´e-Hopf index of the vector field ξ follows from the fact that this vector field is holomorphic, so its index at 0 equals
the dimension of the vector space OC2 ,0 /(z2q , z1p ). 5.3 Remark. There are two special cases which are not envisaged in the previous discussion, these are when p = q = 1 or p > q = 1. These are studied in detail in [202]. When p = q = 1 one has (see §4 in the previous Chapter VII) that if |λ1 | = |λ2 |, then f is not a submersion in a punctured neighborhood of 0 ∈ C2 and the corresponding link L = f −1 (0) ∩ S3ε is not fibred. In fact, this link is not even a  |λ2 | the situation is more interesting. 1-dimensional manifold (see [202]). If |λ1 | = In this case, as well as when p > q = 1, the results of [214] (see VII. 3, 4) imply that f satisfies the Milnor condition at 0, but the methods used there do not say anything about the strong Milnor condition and one cannot decide with those techniques whether or not the map |ff | defines an open book decomposition of S 3ε . In [202] it is proved that they do; this is done by making a careful geometric study of these singularities in a neighborhood of the link. It is proved that in both cases the link L is the Hopf link {z1 z2 = 0} and the map |ff | is, on the complement of L, the projection of a fibre bundle; the fibres are annuli and the monodromy is the identity map.

VIII.6

On singularities of the form f g¯

We know from the previous sections that the links of the singularities V p,q = {z1p z 2 + z2q z 1 = 0 } , p , q ≥ 2 , are Seifert links in S3 , being orbits of the S1 -action on C2 given by: (eit , (z1 , z2 )) → (e−is1 t z1 , e−is2 t z2 ) , 1+p and s2 = where s1 = pq−1 components of the link

1+q pq−1 .

The Hopf link S3 ∩ ({z1 z2 = 0}) gives two

L = V p,q ∩ S3 , the others being a torus link (or knot) of type (p + 1, q + 1). In fact these are the links of type 5 in the classification of Seifert links in [75, 7.3, p. 63]. These links

VIII.6. On singularities of the form f g¯

217

can also be obtained via complex singularities: they are isotopic to the links in S3 determined by z1 z2 (z1p+1 + z2q+1 ) = 0 . So it is natural to ask whether the corresponding Milnor fibrations, and the open book decompositions of S3 , are topologically equivalent. Theorem 5.1 shows that this is not the case. In fact one has that the open book fibration provided by f (z1 , z2 ) = z1p z 2 + z2q z 1 induces the “negative” orientation around the two components of the Hopf link. As a consequence of this fact we obtained (using [200]) that if we let k = gcd(p + 1, q + 1) be the greatest common divisor of these numbers, q+1 f  and if we set p = p+1 k and q = k , then the genus of the fibres of |f | is 1 1 k(kp q  − p − q  − 1) = (pq − 1 − k) , 2 2 while the same method tells us that the genus of the fibres in the holomorphic case is 1 k(kp q  + p + q  + 1) . 2 Similarly, the monodromy for the above open book decompositions has period kp q  − p − q  = k1 (pq − 1), whereas in the holomorphic case it has period kp q  + p + q  . One actually has that the link of f (z1 , z2 ) = z1p z 2 + z2q z 1 is orientationpreserving isotopic to the link of the singularity: (z1 , z2 ) −→ z¯1 z¯2 (z1p+1 + z2q+1 ). This fact, together with A’Campo’s example (c.f. VII.1 above) z1 + z¯2 ) , (z1 , z2 ) −→ z1 z2 (¯ motivated the study in [199] of real singularities in R4 of the form f g¯, with f and g holomorphic functions (C2 , 0) → (C, 0). It is clear that if the function f g¯ has an isolated critical point at 0, so that it satisfies the Milnor condition for having an associated Milnor fibration (see Chapter VII), then the link of this function is L = Lf ∪ Lg , where Lf and Lg are the links of f and g. However, due to complex conjugation, as in the previous case, the components of Lg get the opposite orientation to the one induced by the Milnor fibration of g, thus the corresponding fibrations differ in general. Moreover, that these singularities satisfy the Milnor condition at 0 does not necessarily imply (a priori) that they satisfy the strong Milnor condition and therefore define open book decompositions (c.f. Chapter VII). In fact this type of singularities already appeared in L. Rudolph’s article [215], and his work suggested that given f, g : (C2 , 0) → (C, 0) ,

218

Chapter VIII. Open Book Decompositions of the 3-sphere

holomorphic with no common branch, one has that the real analytic map, f g¯ : (R4 , 0) → (R2 , 0) , has an isolated critical point at 0 iff the link L = Lf ∪ Lg is fibred. This was recently proved by A. Pichon in [199]. She further proved that if this condition holds, then the underlying Milnor fibration is an open book decomposition with binding Lf ∪ Lg , and the projection map is f g¯ |f g¯| in a tubular neighborhood of this link. She also found, using the resolution of the singularity of the holomorphic map-germ f g, a very simple combinatorial argument that allows one to know whether or not the link Lf ∪ Lg is fibred. Using these results she was able to show that a large family of fibrations of Waldhausen links in S3 arise as the Milnor fibrations of real analytic germs f g¯, other than those coming from complex singularities. These results have been recently extended and improved in [201]. In order to explain this we recall (c.f. [75]) that a multilink in S3 is the data of an oriented link L = K1 ∪ · · · ∪ Kl together with a multiplicity ni ∈ Z associated with each component Ki . We denote such a multilink by L = n1 K 1 ∪ · · · ∪ nl K l , and we fix the convention that ni Ki = (−ni )(−Ki ), where −Ki means Ki with the opposite orientation. Now, given a holomorphic map f : (C2 , 0) → (C, 0), we may decompose it into irreducible analytic factors, f =

l

fini ,

i=1

where the ni are all positive integers; in this case the multilink associated with f is Lf =

l +

ni Lfi ,

i=1

where Lfi = fi −1 (0) ∩ Se is,the link of fi . One has a similar decomposition for nj and the multilink associated to g is Lg = the holomorphic map g = m j=1 gj m i=1 −nj Lgj . The link Lf g of f g is Lf ∪ −Lg . 6.1 Definition. A multilink L = n1 K1 ∪ · · · ∪ nl Kl in S3 is fibred if there exists a map Φ : S3 \ L −→ S1 which satisfies the following two conditions: (i) The map Φ is a C ∞ locally trivial fibration. (ii) For each i = 1, . . . , l, there exist a regular neighborhood N (Ki ) of Ki in M \ (L \ Ki ), an orientation-preserving diffeomorphism τ : S1 × D2 → N (Ki ) such that τ (S1 × {0}) = Ki and an integer ki ∈ Z such that for all (t, z) ∈ S1 × (D2 \ {0}) one has:  ni z tki . (Φ ◦ τ )(t, z) = |z| In this case we say that Φ is a fibration of the multilink L.

VIII.6. On singularities of the form f g¯

219

Notice that this is much stronger than just asking the link to be fibred in the sense of Chapter I, which is the first condition in the definition above. Condition (ii) demands that near the binding, the projection map φ takes into account the multiplicity of each component of the multilink. In fact in [201] there is an example showing a link which is fibred in the sense of Chapter I (it is the link D4 ) to which different sets of weights are attached, making it a multilink in various ways. It is shown that with some weights it fibres and with others it does not. One has the following result [201, Theorem 2]: 6.2 Theorem. Let f : (C2 , 0) → (C, 0) and g : (C2 , 0) → (C, 0) be two holomorphic germs without common branches. Then the following two conditions are equivalent: (i) the real analytic germ f g : (C2 , 0) → (R2 , 0) has an isolated critical value at 0 ∈ C; (ii) the multilink Lf g is fibred. Moreover, if these conditions hold, then the Milnor fibration of f g, fg : S3e \ (Lf ∪ Lg ) → S1 , |f g| is a fibration of the multilink Lf − Lg . In fact the theorem in [201] is stronger because it gives a third condition which is equivalent to the two conditions above and is surprisingly easy to verify, making it easy to know whether or not a given multilink is fibred. This improves the results of [199] in two ways: first allowing the multiplicities to be = ±1, i.e., asking only for 0 ∈ C to be an isolated critical value instead of demanding 0 ∈ C2 to be an isolated critical point; and second giving that the projection map of the fibration is f g/|f g| everywhere and not only on a neighborhood of the link. This last improvement is possible thanks to Theorem I.6.4 (also proved in [201]). We remark that Theorem 6.2 allows us to use the machinery of [200] to study the topology of these fibrations, just as in the previous examples in Sections 4 and 5 above.

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Index A’Campo, N., 6, 177, 217 adjunction formula, 107, 117 algebraic knot, 14, 23 Arf invariant, 33, 101, 105, 109, 132, 133 Arnold, V.I., 4, 53, 78, 134, 138, 150 Artin, M., 121 Atiyah, M.F., 4, 106, 138, 150 automorphic form, 56, 58 automorphy factor, 70, 78 Bekka, K., 181 Berndt, J., 4, 138 binary group, 49, 52 binding, 21 blow-up, 110, 200 Brieskorn manifold, 12, 35, 50, 57, 112, 135, 165 Brieskorn, E., 2, 30, 33 Brieskorn-Hamm, 60, 89 C∗ -action, 12, 67 C-regular, 179 canonical bundle, 87, 100, 105, 124 class, 118, 120 divisor, 99, 105, 118 framing, 88 Casson invariant, 135 Castelnuovo’s criterium, 112 Cayley, A., 49 characteristic divisor, 106, 126 characteristic submanifold, 104, 131 Chern class, 70, 87, 94, 104, 105, 117, 125 Chern class, relative, 95 Chern classes, for singular varieties, 97 cohomogeneity, 75, 146, 148

configuration, admissible, 163 contact, of foliations, 169 contacts, variety of, 169 Coxeter, H.S.M., 36 CR structure, 90 CR-line, 144 cusp, singularity, 80, 82 cyclic group, 37 dihedral group, 37 divisor, normal crossings, 111, 201 dodecahedral group, 40 Dolgachev, I., 3, 62, 66, 75, 90, 119, 125 double fibration, 141 double point, 111, 202 Durfee, A., 15, 124, 128, 130 Dynkin diagrams, 119 E + (2), 74 η-invariant, 134 Ehresmann’s lemma, 17 Eisenbud, D., 14 Euclidean geometry, 42 Euler characteristic analytic, 101 of a Seifert fibration, 73 class, 93, 111 relative, 94 number, 68 of a Seifert fibration, 68, 205 rational, 73, 205 exceptional divisor, 111–113, 115 exotic sphere, 2, 30, 32 Fermat polynomial, 140 fibred knot, 21 flag manifold, 141

236 foliation, 78, 79, 143, 147 Fubini-Study metric, 142 Fuchsian group, 72 fundamental domain, 37, 44 G = E(1, 1), 74 geometric genus, 121, 125, 126, 133, 134 Ghys, E., 75 Gorenstein, 75, 76, 82, 87, 116, 125, 131 Gorenstein, numerically, 116, 130 graph decorated, 203 Nielsen, 213 plumbing, 115, 203, 213 resolution, 113, 115, 119 Seifert, 213 Grauert, criterium, 76, 77, 80 Haefliger, A., 164 Hamm, H., 24, 61 Heisenberg group, 74, 79 Hirzebruch, F., 2, 3, 30, 33, 80, 97–99, 101, 128 homotopy sphere, 31 Hopf fibration, 48, 49, 176 Hopf link, 201, 213, 216 Hopf manifold, 165 horizontal Seifert fibration, 206, 207 hyperbolic geometry, 42, 55 icosahedral group, 40 integral closure, 178 intersection matrix, 112, 115 invariant polynomials, 51

Index Lˆe’s example, 23 Leonardo da Vinci, 36 Libgober, A., 140 Lie groups, 3-dimensional, 71, 75, 85 limit set, 143 link (or knot), 12, 13, 16, 22, 204 link, fibred, 22 link, of the singularity, 1, 12, 15–17, 31, 213 link, oriented, 207 Looijenga, E., 6, 24, 123, 125, 177 L´ opez de Medrano, S., 6, 157, 160 Lorentz group, 74 LVM-manifold, 6, 163, 165 M¨ obius group, 43, 55 Massey, W., 138 Meersseman, L., 6, 157, 162 Milnor classes, 30 condition, 19, 175, 178, 217 and finite determinacy, 195 stability of, 194 strong, 178–180, 188 fibration, 1, 18, 22, 24, 32, 171, 191 fibre, 27, 31, 138, 173 number, 25, 27, 28, 121, 134 Milnor, J., 1, 3, 5, 17, 19, 27, 29, 30, 32, 35, 52, 57, 75 monodromy, 21, 31, 139, 181, 187, 193, 213 Morse-Bott function, 147 Morse-Smale, 168 multiplicative sequences of polynomials, 98 Mumford, D., 1, 30, 106, 113

Jacquemard, A., 178 Kervaire, M., 30, 33 Klein, F., 2, 35, 37, 50, 51, 75 Klein, theorem of, 50 Kuiper, N., 4, 138, 163, 176 L-genus, 99 labeled, 57, 78 Laufer, formula of, 121 Laufer, H., 121 Lˆe, D.T., 15, 23, 24, 28, 29, 167, 172

N , 74, 79 N´emethi, A., 136 Neumann, W., 3, 14, 60, 62, 66, 75, 89 Nicolaescu, L., 136 Nielsen Graph, 207 normal map, 142, 145 normal, singularity, 76, 111 O(3), 36 octahedral group, 39 open book, 21, 206, 217

Index open problem, 71, 75, 85, 88, 90, 109, 116, 125, 134, 135, 196–198 Orlik, P., 66 pages, of an open book, 21, 199, 206 ψF,X (z), 167 Pham, F., 25 Pham, join of, 2, 25, 26 Pham-Brieskorn, 2, 11, 16, 25, 32, 60, 167, 191, 192 Pham-Brieskorn, twisted, 182, 188, 191 Pham-Brieskorn, twisted (topology of), 192 Pichon, A., 9, 218 Pinkham, H., 66 plane curve, 13, 14 plumbing, 113, 115 Poincar´e leaf, 163 Poincar´e, H., 1, 42, 44 Poincar´e, homology sphere of, 53, 193 polar variety, 168 polar, hyperpolar action, 147 Pontryagin class, 96, 98 P SL(2, R), 55, 57, 119 P SL(2, R)2 , 81 Puiseux parameterization, 203 quasi-homogeneous singularity, 65, 67, 70, 75, 181 rational Euler number, 70 rational singularity, 121 resolution embedded, 201, 202 good, 111, 202 minimal, 112, 120 of singularity, 110, 111, 115, 200, 203 Riemann-Roch, 97, 100, 101, 107, 121, 125, 126 Rochlin invariant, 131, 132, 135 Rochlin, theorem, 102, 105, 107, 109 Ruas, M.A., 181 Rudolph, L., 217 S 3 as a Lie group, 47 S 3 , finite subgroups of, 47, 49 Saito, K., 71 Santos, R.N.A., 181

237 Schwarz, H.A., 35, 42, 44 Seiberg-Witten invariant, 125, 135 Seifert fibration holomorphic, 68 fibre, 67 exceptional, 67 graph, 204 invariants, 12, 120 reduced, 67, 68, 73, 205 link, 204, 206 manifold, 12, 58, 67, 73, 120 self-intersection, 111 Siegel domain, 159, 160, 162, 163 Siegel leaf, 157, 163 signature, 33, 97, 99, 103, 128 signature defect, 91, 128, 130
SL(2, R), 55, 57, 72, 77, 119 Smale, S., 30 smoothing, 29, 77, 123, 125, 128 SO(3), 36, 148 finite subgroups of, 37, 45 SO(n + 1, R)-action, 145 Sp(1), 49 spherical (elliptic) geometry, 42 spin structure, 87, 103, 105 canonical, 88 spinc structure, 104, 105, 126 Stiefel-Whitney class, 95, 103, 105 relative, 96, 131 SU (2), 48, 72 Sullivan, D.P., 43 Teissier, B., 14, 15 tesselation, 44 tetrahedral group, 38 Thom, R., 6, 97, 169 Thurston, W., 89 Todd genus, 101, 109, 125 torus knot, link, 13, 204, 213 transform, proper, strict, 110, 201 transform, total, 110 trefoil knot, 30, 201 triangle group, 37, 42, 44, 45, 57 full, 44 uniform subgroup, 71

238 vanishing cycle, 28 Vassiliev, V., 138 Verjovsky, A., 6, 157 vertical divisor, 121 virtual degree, 70 w.h.c., weak hyperbolicity condition, 161, 163 Wagreich, P., 66 Wahl, J., 123 Waldhausen, F., 204, 206 Wall, C.T.C., 138, 160 weighted homogeneous, 66, 189 Winkelnkemper, E., 21 Zariski, O., 137 Zariski-Lipman conjecture, 116

Index